feat(agents-md): contribute homeboy CLI section when on PATH

[chubes4]

Summary

When the homeboy Rust CLI is on PATH, AGENTS.md gains a ## Homeboy section that maps the verbs agents reach for and the repo-level rules they must respect.

Adds:

• inc/Homeboy.php — capability helper. Single command -v homeboy probe, memoized, hard-gated on Environment::has_shell().
• SectionRegistry contributor at priority 35 (between wordpress-source and multisite).
• tests/smoke-homeboy.php — 5 assertions across no-shell / empty-PATH / synthetic-on-PATH / memoization / reset_cache.

Design

Sits on top of #57 (DMC = WP↔runtime bridge). Mirrors the house style of ## Data Machine and ## Abilities: terse map, bold sub-labels, one discoverability hint at the end. The CLI is its own source of truth — homeboy --help enumerates everything; this section calls out the verbs agents should reach for and the repo rules that protect release automation.

Section content (preview)

## Homeboy

`homeboy` is a Rust CLI on this host. Every verb runs the same locally as in CI.

**Quality:** `homeboy audit | lint | test | refactor`

**Git:** prefer `homeboy git changes | status | commit | push | pull | tag`
— auto-baselines, structured output, `--json` bulk. One-off reads
(`git diff`, `git show`, `git blame`) stay on raw `git`.

**Perf + envs:** `homeboy bench` for pinned benchmarks, `homeboy rig` for
reproducible dev environments.

**Repo rules** (when `homeboy.json` is present):
- **NEVER edit `CHANGELOG.md`** — generated from conventional commits at
  release time.
- **NEVER hand-bump version strings** — `feat:`/`fix:`/`BREAKING CHANGE`
  drive semver; Homeboy rewrites version targets in `homeboy.json`.

Run `homeboy --help` for the full verb list. Operator verbs
(`release`, `deploy`, `fleet`, `ssh`) only on explicit ask.

Tests

$ php tests/smoke-homeboy.php
✓ empty PATH: is_available() returns false
✓ absent: result is memoized
✓ synthetic on PATH: is_available() returns true
✓ memoization: result sticky after PATH change
✓ reset_cache: re-probe sees current state
OK

Pure-PHP, no WP bootstrap. Deterministic via PATH override.

AI assistance

• AI assistance: Yes
• Tool(s): Claude Code (Sonnet 4.5)
• Used for: Drafted helper, contributor, smoke, and this body. Iterated section content twice — first draft over-enumerated verbs (mentioned release, deploy, fleet as primary; missed bench and rig); second draft over-explained ("prefer-over-raw-git" essay paragraphs). Chris pushed back on both ("you overcorrected"; "match the house style — minimal, clean map") and the section was tightened to mirror ## Data Machine density.