feat(init): add roar init agents to install agent guidance

[christophergeyer]

Summary

• Adds roar init agents subcommand that installs agent-facing guidance:
  • ~/.claude/skills/roar/SKILL.md — Claude-specific skill, version-stamped
  • ./AGENTS.md section — cross-agent, fenced inside <!-- roar:begin -->...<!-- roar:end --> markers for idempotent re-runs
• Flags: --skill / --project (limit scope), --dry-run, --force, --check (CI-friendly drift detection), --path
• Backward compat: bare roar init still works. init becomes a click group with invoke_without_command=True.
• Templates ship under roar/cli/templates/agents/ and are bundled in sdist + wheel via maturin include list.

Behavior

• Default: install both targets.
• Skill on hand-edited file: refuses to overwrite (no managed marker found); --force overrides.
• AGENTS.md on existing file: appends a fenced block; user content outside the block is never touched.
• --check: exits nonzero if installed config is missing or out of date — suitable for pre-commit / CI.

Test plan

• 24 new unit tests covering pure update functions + CLI integration (idempotency, dry-run, check, force, content preservation, backward compat)
• Full unit suite passes (561 passed, 1 skipped)
• Lint clean (ruff check)
• Manual smoke test: install, re-run (noop), --check (passes after install)
• Manual: verify wheel contains templates after pip install from built wheel

Notes

• MCP integration intentionally skipped per discussion — revisit when there's evidence agents would benefit from structured lineage queries vs. parsing roar show text.

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