chore: ignore .agents/ (per-user glab-installed skills)

glab skills install <name> places bundled agent skills in .agents/skills/ at the repo root by default. That's a per-user install destination (parallel to ~/.agents/skills/ with --global), not shared source — different contributors will install different skill sets, and any skill can be reinstalled at any time from the bundled definitions in glab itself.

Ignoring keeps the tree clean without dictating one contributor's skill choices to the rest of the project. Same treatment we already give .claude/settings.local.json.

Rollback path

If we later want a shared skill baked into the repo (e.g. one specific skill everyone should have available), we can carve it out with !.agents/skills/<name>/ — same pattern .vscode/ already uses for launch.json.