refactor: delete the dead messaging island + add knip to CI (#92)

[TortoiseWolfe]

Fixes #92. Resolves the messaging cluster of #90.

Four modules (719 LOC) plus three test files formed a closed island — importing each other, with nothing outside importing any of them. Verified beyond static imports: no dynamic import() with a matching specifier, no string-built paths.

useConversationRealtime.ts        ← no importers at all
  ├── useConversationRealtimeSync.ts  (:24)
  │     └── decrypt-message.ts        (:22)
  ├── decrypt-message.ts              (:23)
  └── decryption-cache.ts             (:19)

Born dead, not orphaned. useConversationRealtime was in the initial commit, and page.tsx in that same commit already had the full-refetch loadMessages. git log --all -S over src/app/ and src/components/ returns empty across all history (control query confirms the search works). There is nothing to roll back to.

It had already cost real time twice. On 2026-03-24, a5d4e3d added 55 lines of polling fallback into the dead hook to fix CI flake; it fixed nothing, and 812cead hand-copied the logic into page.tsx twelve hours later — its title says "was dead code". More recently it produced a false root cause on #69, which named clearDecryptionCaches() as the live cause of message loss. That function cannot execute.

Changes

• Delete the four modules + three test files (c1c6217)
• Remove onKeysChanged/notifyKeyChange + five call sites + its one test (30abb17) — kept a separate commit because it touches a live service, so it can be reverted independently
• Retarget the four documentation sites that referenced the island (d0d1d3d) — including SECURITY-ARCHITECTURE.md, which described the never-wired cache as a live DoS mitigation
• Add knip with a baseline and a CI gate (325d114)

No behaviour change

Both harvest items in the issue turned out moot, which makes this a pure deletion:

• upsertMessage's id-based dedupe cannot replace page.tsx's content heuristic while optimistic rows carry optimistic-<ts> ids and server rows carry UUIDs. Blocked on Messaging live path: 5 round-trips per event, lost-update race, content-based dedupe #91.
• The live path already has all three failure strings (message-service.ts:767, :859, plus EncryptionLockedError) — the issue's claim that it had only one was wrong.

knip

Baselined at 37 findings so the gate is green on arrival and fails only on new unreferenced files. The baseline holds two deliberately-distinct categories, labelled in knip.jsonc: 16 one-line barrels re-exporting live components (an import-style choice, out of scope per #82), and 21 genuine orphans for #90 to burn down. It is a burn-down list, not an allowlist.

.storybook/mocks/** is ignored for a different reason — reached via path.resolve aliases knip cannot follow.

Mutation-checked: planting an unreferenced file under src/lib/ turns the gate red and names it; removing it returns exit 0.

Verification

Check	Result
type-check	clean
lint	0 errors (1 pre-existing warning outside this diff)
Unit suite	385 files, 4015 passed, 0 failed
Test-count delta	exact match — 4047→4015 is 32; deleted files held 15 + 9 + 7 + 1 = 32
Coverage	68.34 / 60 / 70.79 / 69.16 vs 58 thresholds — passes
knip	exit 0, mutation-proven

The coverage concern raised in the issue (deleting well-covered LOC might trip the 58% gate) was flagged there as unmeasured. Measured: it does not.

Note on ordering

pnpm build could not be verified locally on this branch — that is #93, fixed in #94, which should land first. CI verifies the build here.

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