fix: scope agent rule discovery to instruction paths

[t-gok]

Summary

Rule discovery currently scans all Markdown beneath .claude, .codex, and .gemini, loading skill bodies, references, commands, and cached plugin content into the initial prompt as rules. A synthetic workspace with 100 skills produces a 3.17 MB prompt even though skill discovery already exposes metadata separately.

Restrict rule discovery to each agent's rules/ directory and explicit instruction files. The same fixture produces about 20 KB while retaining all 100 skills in discovery. Preserve instruction bootstrap order, namespaces, and selectors, and include Gemini's style guide once.

Compatibility: other rule Markdown under these agent directories must move into rules/ or .agents/rules/. The search-path documentation includes migration instructions.

Validation

• Regression coverage for all three agents, large skill trees, excluded reference/command/cache content, skill bootstrap isolation, explicit instructions, instruction bootstrap order, namespaces, and selectors.
• go test -race ./pkg/...
• Full go test ./... in a network-disabled local container with an empty default home, avoiding unrelated installed user skills.
• make lint; unrelated baseline formatting changes excluded.

Based on upstream c772470.