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>

AGENTS.md

@@ -4,6 +4,8 @@ This is a **Turborepo + pnpm monorepo** organized by vertical features. Each fea
 
 > **Vocabulary:** Every cross-cutting term used in this repo (feature, use case, manifest, slice, conformance, dispatch, etc.) is defined in [`docs/glossary.md`](./docs/glossary.md). When in doubt about what a term means **here**, check the glossary first — it's the single source for shared vocabulary between humans and agents.
 
+> **Commits:** Every commit message follows [Conventional Commits](https://www.conventionalcommits.org/): `<type>(<scope>): <imperative subject>` (≤72 chars). Types: `feat | fix | docs | style | refactor | test | chore | perf | ci | build | revert`. Use `!` for breaking changes. The sandcastle implementer + reviewer prompts enforce this; agents authoring autonomously MUST honor it.
+
 ## Agent-driven development
 
 This template assumes agents (Claude, Codex, etc.) will author most feature work. The orchestration substrate is [Sandcastle](https://github.com/mattpocock/sandcastle) — see [ADR-019](./docs/decisions/adr-019-sandcastle-for-agent-orchestration.md). Day-to-day entry points: