feat: add OpenCode planner and mechanical agents

[Grzanka777]

Summary

• add the OpenCode planner agent as the structured workflow entry point
• add the OpenCode mechanical agent for deterministic low-judgment operations
• update Agent Pack manifest, capability matrix, README, and roadmap
• add planner, mechanical, coexistence, and documentation tests

Validation

• ruff format --check: pass
• ruff check: pass
• mypy: pass
• pytest: 64 passed
• git diff --check: clean
• final independent review: PASS
• staging audit: PASS

Safety

• no shared contract changes
• no VERSION change
• no Oracle Wisdom changes
• no Codex adapter implementation
• no Brain write