refactor(hooks)!: remove 4 low-value hooks (P2.1)

[klpanagi]

Summary

Removes heaviness from the hook registry — 1 ghost schema entry + 3 low-value implementations.

BREAKING CHANGE: disabled_hooks entries for the 4 deleted names now fail Zod validation. Remove them from matrixx.jsonc.

Changes

• HookNameSchema 65→61 (src/config/schema/hooks.ts): delete grep-output-truncator (ghost, 0 LOC, already covered by tool-output-truncator's TRUNCATABLE_TOOLS), question-label-truncator (63 LOC, PreToolUse hot path), mcp-startup-notification (37 LOC, startup toast), directory-readme-injector (122 LOC, duplicate README injection with FS access storm)
• Deleted: src/hooks/question-label-truncator/, src/hooks/mcp-startup-notification/, src/hooks/directory-readme-injector/ (6 files) + 3 test dirs (~470 LOC) — 26 files -764/+22
• Wiring cleaned: src/hooks/index.ts (3 exports), create-session-hooks.ts (un-gated truncator + gated mcp), create-tool-guard-hooks.ts, tool-execute-before/after.ts, event.ts, hook-mutation-classification.md, tool-execute-before.bench.ts (16→14 calls), src/hooks/AGENTS.md
• Docs: docs/configurations.md enumeration 65→61, docs/features.md, docs/context-management.md
• Schema: assets/matrixx.schema.json regenerated via bun run build:schema

Rationale

Hook	Cost	Value	Decision
grep-output-truncator	0 runtime (ghost)	pollutes schema/autocomplete	pure delete
question-label-truncator	PreToolUse on every tool call	30-char label slice for single tool	delete, TUI handles overflow
mcp-startup-notification	session.created once	toast, failures already queryable via getMcpStartupFailures	delete
directory-readme-injector	PostToolUse + access/readFile per file read	duplicates directory-agents-injector	delete, factory kept

Verification

• bun run typecheck — 0 errors
• bun run lint — 960 files, 0 errors
• bun run build — 1140 modules, ESM+CLI+schema green
• grep -c hooks.ts → 61, orphan rg for 4 names → 0 matches in src/tests
• ls 3 hook dirs → No such file, survivors intact

Follow-up

P2.2 (context-window consolidation), P2.3 (evolution gating), P2.4 (skill consolidation), P3 remain. manager.ts 1749 LOC deferred (needs Oracle facade plan).

Closes P2.1