Add Homeboy Codebox canary verification

[chubes4]

Summary

• Add an explicit opt-in scripts/verify-homeboy-codebox-canary.sh helper that builds a bounded read-only Homeboy Codebox dispatch, requires provider secret env names, and validates terminal status/log/changed-files/patch evidence.
• Document the stronger verify-homeboy-codebox-canary overlay and explain how it differs from datamachine_code_homeboy_available.
• Cover canary command construction and validation with a fake-Homeboy test, and fix the WP Codebox version-header regex to be POSIX-portable after the existing subtree test exposed the issue.

Verification

• bash tests/homeboy-codebox-canary.sh
• bash tests/wp-codebox-subtree.sh
• for test in tests/*.sh; do bash "$test"; done; for test in tests/*.mjs; do node "$test"; done
• Live canary: proof-wp-coding-agents-issue-190-20260607-002 succeeded with changed files 0 and patch bytes 0.

Closes #190.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Implemented the opt-in Homeboy Codebox canary helper, verification docs, unit coverage, live canary validation, and PR preparation under Chris's requested requirements.