docs: surface Conventional Commits requirement across agent entry points

The implementer + reviewer sandcastle prompts already enforce
conventional commits, but the convention was buried in those prompt
templates — agents not running through sandcastle (e.g. interactive
Claude sessions, ad-hoc fixups) had no visible signal that the format
is non-negotiable.

Four visibility surfaces now carry the signal:

  - CLAUDE.md Key Conventions: new top bullet stating the spec
    (<type>(<scope>): <subject>), the full type list, breaking-change
    syntax (!), and three example commits
  - AGENTS.md preamble: parallel callout alongside the vocabulary
    and quality-gates notes
  - .claude/hooks/session-start.sh: one-line reminder in the boot
    pointers (every session now sees it at start)
  - .claude/hooks/prompt-context.sh: new 10th keyword group fires
    when a user prompt mentions commit/message/changelog/conventional,
    injecting the full spec into the turn's context

Conventional Commits spec: https://www.conventionalcommits.org/

Verified: the prompt-context hook smoke-tests green when a prompt
containing "commit message" is piped in — it emits the conventional
commits pointer.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -90,6 +90,7 @@ See `docs/guides/coverage.md` for the cookbook and ADR-020 for the full rational
 
 ## Key Conventions
 
+- **Conventional Commits (non-negotiable)** — Every commit message MUST follow the [Conventional Commits](https://www.conventionalcommits.org/) spec: `<type>(<scope>): <imperative subject>` (≤72 chars). Types: `feat | fix | docs | style | refactor | test | chore | perf | ci | build | revert`. Use `!` after type/scope for breaking changes. Body explains WHY if non-obvious. Examples: `feat(auth): hash password before persisting`, `test(blog): assert article not found error`, `refactor(docs)!: consolidate scaffolding into guides`. The sandcastle implementer + reviewer prompts both enforce this; agents authoring commits autonomously MUST honor it.
 - **Relative imports in `src/`** — Source files use relative paths (`../repositories/...`), not `@/` alias
 - **`@/` alias in tests** — Test files (`*.test.ts`) use `@/` to import from `src/`
 - **`vitest.config.ts`** — Every package must define `resolve.alias: { "@": path.resolve(__dirname, "./src") }`