docs: track AGENTS.md and symlink CLAUDE.md to it

Mole is now developed with both Claude Code and Codex. Both expect their
own guide file (CLAUDE.md / AGENTS.md). Previously both were gitignored
and maintained as local files, which meant new checkouts and other agents
started without project context.

Track AGENTS.md as the single source of truth, symlink CLAUDE.md to it so
Claude Code reads the same content, and keep *.local.md ignored for
machine-specific overrides.