Add Markdown custom agent selection

[cnjack]

What changed

• replace unused JSON custom-agent definitions with layered *.agent.md files under ~/.jcode/agents/ and <project>/.jcode/agents/
• require name and description frontmatter plus a non-empty Markdown instruction body; support an optional role model
• apply deterministic duplicate handling: first valid filename in a layer wins, and project roles override user roles
• use the same effective role for top-level sessions and delegated subagents, workflows, and team members
• add jcode --agent, a conditional Web/Desktop Agent picker, active-agent display, localized switch notices, and session persistence/replay
• keep ACP on Default for new sessions while restoring a saved custom role on load/resume
• add internal and published documentation plus loader, session, API, UI, routing, and compatibility tests

Why

Custom agents already existed as an unused JSON-only delegated-role mechanism, but there was no top-level selection or visible current-agent state. The new Markdown format matches the repository's instruction-oriented workflows and follows the useful Codex/Grok semantics: role instructions are reusable, an optional role model can override selection, and runtime permissions remain inherited from the caller.

User/developer impact

When valid custom agents exist, Web/Desktop shows Default plus every effective role. CLI users can select one with jcode --agent <name>. Agent changes survive resume and appear in the timeline. Custom Markdown cannot add tools, bypass approval, or broaden the active mode/sandbox.

This intentionally removes inline/legacy JSON agent definitions; the feature was not yet in use.

Validation

• go test ./...
• go test -race ./internal/config ./internal/session ./internal/web ./internal/command
• make lint
• make build
• pnpm --dir packages/jcode-ui test
• pnpm --dir web test
• pnpm --dir site build
• pre-push hook: Go build, vet, golangci-lint, and full Go test suite

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 45774ccd-1a0f-4a9f-a638-64ade31b8db1

📥 Commits

Reviewing files that changed from the base of the PR and between 7fd8fb4 and f63ce14.

📒 Files selected for processing (55)

• cmd/jcode/main.go
• go.mod
• internal-doc/custom-agents.md
• internal/command/acp.go
• internal/command/custom_agents.go
• internal/command/custom_agents_test.go
• internal/command/interactive.go
• internal/command/web.go
• internal/config/agent_roles.go
• internal/config/agent_roles_test.go
• internal/config/config.go
• internal/session/agent_roundtrip_test.go
• internal/session/history.go
• internal/session/session.go
• internal/team/manager.go
• internal/tools/flow_spawn.go
• internal/tools/subagent.go
• internal/tools/subagent_model_routing_test.go
• internal/tools/subagent_test.go
• internal/tools/team_tools.go
• internal/tools/team_tools_test.go
• internal/tui/messages.go
• internal/tui/session_helper.go
• internal/tui/session_helper_test.go
• internal/tui/update.go
• internal/web/agents.go
• internal/web/agents_test.go
• internal/web/chat.go
• internal/web/engine.go
• internal/web/server.go
• internal/web/sessions.go
• packages/jcode-ui/src/product/ChatInput.test.tsx
• packages/jcode-ui/src/product/ChatInput.tsx
• packages/jcode-ui/src/product/host.ts
• packages/jcode-ui/src/product/index.ts
• packages/jcode-ui/src/product/strings.ts
• packages/jcode-ui/src/product/types.ts
• packages/jcode-ui/src/styles/composer2.css
• site/docs/overview/custom-agents.md
• web/src/App.tsx
• web/src/app/composerHost.ts
• web/src/app/store.ts
• web/src/app/wsBridge.ts
• web/src/components/AuthGate.tsx
• web/src/components/ChatView.tsx
• web/src/components/CommandPalette.tsx
• web/src/components/SetupView.tsx
• web/src/i18n/locales/en.ts
• web/src/i18n/locales/ja.ts
• web/src/i18n/locales/ko.ts
• web/src/i18n/locales/zh-Hans.ts
• web/src/i18n/locales/zh-Hant.ts
• web/src/lib/api.ts
• web/src/lib/types.ts
• web/src/lib/ws.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/custom-agent-markdown-selector

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