fix: generate .agents directory at setup time instead of shipping duplicates (#308)

Removes 14K+ lines of committed generated Codex skill files from git.
.agents/ is now gitignored and generated at setup time via
`bun run gen:skill-docs --host codex`. Updates CI workflow to validate
generation instead of checking committed file freshness.

Co-authored-by: cskwork <cskwork@users.noreply.github.com>
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitignore

@@ -3,6 +3,7 @@ node_modules/
 browse/dist/
 .gstack/
 .claude/skills/
+.agents/
 .context/
 /tmp/
 *.log