fix(subagent-prompts): instruct subagents to read CLAUDE.md for project conventions

[arittr]

Summary

• Adds a one-liner to each of the three subagent dispatch templates instructing subagents to read CLAUDE.md (or AGENTS.md) before starting work
• implementer-prompt.md: "Read CLAUDE.md... its conventions are binding on your work" in the Before You Begin section
• spec-reviewer-prompt.md: "verify the implementation follows its conventions" in the Your Job section
• code-quality-reviewer-prompt.md: "use its conventions as review criteria" before the review checklist

Why

Subagents spawned via the Agent tool start with a fresh context — no CLAUDE.md pre-loaded. They pattern-match existing code well, but miss cross-cutting conventions only documented in CLAUDE.md (test location, DB mocking policy, architectural boundaries, error format conventions).

RED/GREEN tested across multiple scenarios including legacy codebases with conflicting patterns. Without the fix, subagents copy legacy anti-patterns (if/else, try/catch, console.log, argument mutation). With the fix, subagents read CLAUDE.md first and follow its conventions over conflicting existing code.

Test plan

• RED tested: subagents without fix copy legacy patterns from existing code
• GREEN tested: subagents with fix read CLAUDE.md and follow its conventions over conflicting code
• Verified fix works with empty projects (CLAUDE.md only, no existing code)
• Verified fix works with conflicting legacy code (no disclaimer comments)
• Manual verification in a real SDD workflow

Closes #793
Related: #804

🤖 Generated with Claude Code

[arittr]

This change is part of the following stack:

• docs(codex-tools): add named agent dispatch mapping for Codex #900
  • fix(writing-skills): correct false 'only two fields' frontmatter claim (#882) #903
    • fix(subagent-prompts): instruct subagents to read CLAUDE.md for project conventions #899 ◀
      • feat: Codex App worktree and sandbox compatibility #901
        • feat(codex): add Codex plugin marketplace artifacts and docs #904

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