fix: bake the resolved homeboy path into AGENTS.md guidance

[chubes4]

Fixes #575.

Problem

The emitted guidance block resolved homeboy by walking getenv('PATH') at AGENTS.md compose time. AGENTS.md is recomposed by whichever process triggers it — PHP-FPM, cron, a plugin upgrade, a WP-CLI call with a trimmed environment — and none of those carry an interactive PATH.

Observed live on a host with homeboy installed and executable: AGENTS.md silently dropped from 81 lines / 5 sections to 54 / 4. The deleted section is the one instructing the agent to route tracked work through Homeboy, so the failure quietly reverts agent behavior.

Change

Resolve the absolute path once at sync time, where guidance_applies() already runs type -P homeboy, and bake it into the block. The compose-time check becomes a single is_executable().

The PATH walk is deleted, not guarded — about 20 lines of PHP removed.

The #254 live gate is preserved

Still re-checked at every compose, so a host that loses the binary stops emitting the section with no wp-coding-agents sync. The existing Homeboy guidance is removed when Homeboy becomes unavailable assertion passes unchanged.

Verification

bash tests/agents-md-guidance.sh — all assertions pass.

Three new assertions, and I confirmed they go red against the old implementation rather than assuming they were meaningful:

FAIL Homeboy guidance bakes the absolute binary path
FAIL Homeboy guidance still resolves the binary from the ambient PATH
FAIL Homeboy guidance still composes with an empty PATH
  got: {"registered":false,"renders":false}

The third executes the generated mu-plugin under putenv('PATH=') — the exact live condition — and asserts the section still registers and renders.

Emitted PHP linted with php -l.

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🤖 Authored by chubes-bot (Claude). Finalized outside Homeboy under explicit operator authorization: Cook could not admit the task on this host because its opencode route is missing AI_PROVIDER_OPENAI_CODEX_* credentials. Bounded recovery was attempted first — --preview, native homeboy worktree create, backend rotation, homeboy upgrade 0.360.4 → 0.367.9, and homeboy daemon recover. Work was done in the linked isolated worktree wp-coding-agents@fix-issue-575-homeboy-guidance-path.