feat(cms-mode): host the CMS block editor in the side panel

[tlgimenes]

Summary

Two things, and the second one is why the first became interesting.

1. CMS mode gets a real home. Fast Preview projects rendered an inert chat panel — a "coming soon" notice — while the block editor was squeezed into a 31% pane nested inside the Preview tab. Three columns, and the widest one did nothing. The side panel now hosts the CMS editor and the preview canvas takes the whole main panel. Fast Preview is renamed CMS mode throughout.

2. The gate becomes per-branch, which makes the two modes composable instead of exclusive. resolveCmsMode(metadata).active short-circuited the entire sandbox proxy to runner: null for every branch of a CMS project — so a CMS project could never boot a pod, and vibecoding was unreachable. The gate now splits: resolveCmsMode is the project capability, and the new resolveCmsModeForBranch narrows it with "does this branch have a sandbox?". Provisioning one moves that branch onto the daemon while its siblings stay sandbox-less.

Sandbox-only projects are unchanged.

Why per-branch is the whole design

The two editors write to different layers of the same branch:

CMS write   → patchDecofile → GitHub API → a commit on the branch head
Vibe write  → POST /sandbox/…/write     → a file in the pod's working tree

Gating on the project gives a branch two writers: the CMS committing to a head that the pod — holding uncommitted work — can no longer see. The agent's next commit then clobbers or conflicts.

Gating on the branch gives it exactly one. Every CMS hook already had both substrates wired (use-save-block, use-delete-block, use-decofile, use-section-preview-base each branch on cmsModeActive), so following the branch means a CMS edit on a pod-backed draft lands in the working tree next to the agent's edits. There is no divergence left to warn about — the fix is deleting a wrong condition, not adding a reconciliation UI.

The switch

• Sandbox-less draft → the chat composer offers "Start coding" (SANDBOX_START) instead of a locked input.
• Draft with a pod → both CMS and Chat toggles, not one or the other.
• Not one-way: the CMS panel keeps working, now writing through the pod.

Notable

• SidePanelKind widens to "chat" | "cms" across all six sites that hardcoded the literal. Three failed silently — the router zod schema rejected ?sidepanel=cms, and thread-layout memory + chat navigation dropped it — so the panel would have vanished on navigation with no error.
• Retiring the nested pane needed more than emptying it. defaultSize applies at mount, so gating only its content left a 210px dead column. It has to not render.
• The CMS tour anchor moves to the new toggle — that anchor is the tour's readiness gate, so leaving it on a removed button would have stopped the tour starting.
• The branch flips at the click, not when the metadata refetch lands: cmsModeActive also counts a pending start and the seeded preview URL. In that window the pod is already cloning the head, so a CMS write routed there could miss the clone and vanish silently; routed to the not-yet-reachable sandbox it fails visibly.
• hasVmForBranch is kind-agnostic on purpose, and branchHasSandbox falls back to the thread row — a thread-scoped branch records its pod there, and missing it would hand a pod-backed branch back to the head-committing path.

Deliberate: the write path still writes the legacy key

resolveCmsMode reads cmsMode first and falls back to fastPreview, but writers still write fastPreview. Flipping the write before every reader ships would 404 the CMS for any newly-toggled project, and the metadata object is .loose(), so a premature cmsMode write type-checks and fails only against a real server. An e2e proves cmsMode alone opens the gate.

Testing

fmt · check · lint · knip clean.

• New e2e: cms-mode-branch-gate.spec.ts — asserts over HTTP that git/status is GitHub-backed until a sandbox is recorded for the branch, that recording one takes that branch off the GitHub path, and that its sibling is unaffected. A project-level gate fails the third assertion.
• The CMS-project fixture is promoted out of decofile-api.spec.ts into fixtures/cms-project.ts, so both suites share one definition. (This was listed as a follow-up on an earlier revision of this PR; it's done.)
• 12/12 pass for both specs against a clean Postgres locally.
• New unit tests cover resolveCmsMode and resolveCmsModeForBranch.
• Unit-test counts baselined against a stashed pristine tree: identical failures with and without these changes (environmental — cross-file DOM-setup collisions), plus the new passes.

Verified live in the native app: toggle renders as CMS, panel mounts with the real section list, ?sidepanel=cms survives a full reload, nested pane and old Edit content toggle both gone.

Known gaps

• The mode transition has not been clicked through by a human. It's covered by the e2e above and typechecks, but nobody has watched CMS draft → Start coding → tabs/console/chat appear → CMS panel still editing through the pod. Worth doing before merge.
• header-actions.tsx carries ~100 lines of dead CMS handling. VirtualMcpHeaderInfo has routed CMS projects to CmsHeaderActions since feat(fast-preview): smart primary action button for CMS mode #6054, so those branches were already unreachable before this PR. Left alone rather than widening the diff.
• sandbox-events-context.tsx still reads the project-level gate for its blocks-save reload heuristic. Cosmetic only (the decofile refetch inside is separately gated on the dev server), and the legacy semantics there predate the sandbox-less rework.

🤖 Generated with Claude Code

---

Summary by cubic

Hosts the CMS block editor in the side panel and renames Fast Preview to CMS mode. The CMS/Vibecoding switch now governs the entire workspace and gates per branch, so one draft can start vibecoding without affecting siblings.

• Consolidates gating in packages/shared as resolveCmsMode (capability) and resolveCmsModeForBranch (per-branch runtime). API routes (decofile, sandbox-proxy) and web read the same rule; writers still persist the legacy fastPreview key.
• Replaces paired toggles with a split button (CMS | Vibecoding). The body collapses/reopens the current mode; the caret selects the mode. Selection persists as ?mode=cms|vibecoding and is remembered per thread; CMS projects default the side panel to CMS.
• Widens SidePanelKind to "chat" | "cms" and sanitizes untrusted values with parseSidePanelKind across router search, layout memory, task switch, and chat navigation.
• Retires the inline blocks pane on CMS projects via resolveEffectiveEditingMode; CMS editing lives in the side panel in both modes. Section previews use buildCmsDraftUrl; preview display uses cmsModeActive/cmsModeReady.
• Mode changes update preview origin, view tabs, and console together. CMS hides Code when no sandbox exists; the bottom drawer mounts only with a sandbox and manages its own collapsed/expanded state (terminal visibility prefs and provider removed).
• Chat on sandbox‑less drafts offers “Start vibecoding” and flips the per‑branch gate immediately; once a sandbox exists the CMS panel keeps working and writes through the pod. The blocks panel warns when the pod has uncommitted work (countLocalWork).
• Tests: new e2e cms-mode-branch-gate.spec.ts; shared CMS fixtures; unit tests for gates, editing‑mode downgrades, preview display; side‑panel toggles; layout memory and task‑switch restore; sandbox git‑API counts.

Migration

• Replace @/sdk/fast-preview with @/sdk/cms-mode; rename buildFastPreviewDraftUrl to buildCmsDraftUrl; update resolveSectionPreviewBase to accept cmsModeActive.
• Accept and persist "cms" in side‑panel state; use parseSidePanelKind for untrusted input. Persist the selected mode via the mode query param (cms | vibecoding) and restore it per thread.
• Use resolveCmsModeForBranch for runtime gating and resolveCmsMode for capability checks (API and web).
• Remove terminal visibility preferences, overrides, and any TerminalVisibilityProvider usage; do not assume the Code tab or console exists on sandbox‑less branches.
• Update copy to “CMS mode” and adopt the split‑button switch (grid mark for CMS, code mark for vibecoding).

^{Written for commit b2d8dd3. Summary will update on new commits.}