chore(tooling): Add dotagents configuration (#5230)

* meta: Add dotagents configuration

Sets up dotagents to manage AI agent skills declaratively via agents.toml.
Previously committed skills are now managed by dotagents and gitignored.

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

* meta: Keep custom skills alongside dotagents-managed ones

Use a nested .gitignore in .claude/skills/ to ignore dotagents-managed
skills while preserving repo-specific custom skills (create-java-pr, test).

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

* Declare built-in skills

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