docs(agents): tighten code comment rules against stale anchors

[sha1n]

Summary

Rewrites the Code Comments section of the global agent instructions to address a recurring problem: agents adding comments anchored to line numbers, positions, and other volatile references that go stale as the code moves.

Changes

• Flips the default from "comments must earn their place" to "no comment by default", so the burden is explicitly on adding one.
• Adds an enumerated never-anchor list (line numbers, position words, other modules' internals, versions/tickets, the change itself). The previous abstract phrasing wasn't binding — an agent doesn't recognize // see the guard above as an instance of "avoid volatile references".
• Exempts public-interface doc comments (JSDoc, docstrings, godoc) from the default, so the stricter rule doesn't suppress API documentation on exported surfaces.
• Kept to ~195 words. This file loads into every session, so section length competes with the other rules for attention.

Two clauses to watch in practice: the above/below bullet may misfire on legitimate prose, and the doc-comment exemption is the most stretchable escape hatch.