docs: add Agent Skills section for rill init --agent - #9829

• Adds a new "Agent Skills" docs section under developers/agent-skills/ with four pages: overview, installation, updating, and a skills reference, documenting the skills generated by rill init --agent
• Recommends rill init --agent all as the primary command across the docs, with tool-specific options (claude, cursor, agentsmd) as alternatives
• Cross-links the new section from the Agentic Quickstart, the AI Features guide page, and the external IDE page
• Fixes a stale rill init --template reference on the IDE page (the flag is --agent)

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!