Add container/member doc-comment guideline to AGENTS.md

[warp-agent-staging]

Description

Adds a new guideline to the Comments section of AGENTS.md, based on feedback from a Slack thread: an agent had added a struct-level doc comment that enumerated and explained several of the struct's fields, while those same fields also carried their own doc comments repeating the same explanation (see app/src/terminal/event.rs in PR #15162, the UserBlockCompleted struct).

The new bullet, **Container docs describe the whole, member docs describe the parts**, states that a member's own doc comment is where that member gets explained, and a container's item-level doc comment must not enumerate or re-explain its members. It generalizes the existing **Single-source of documentation** bullet to the container/member relationship, and is placed directly after it to keep related guidance together.

This is a docs-only change to AGENTS.md; no other files are touched.

Testing

This is a Markdown documentation change with no executable code, so no automated or manual testing applies. I confirmed the new bullet's lines stay within the section's documented ~100-column wrap, consistent with the surrounding bullets.

• I have manually tested my changes locally with ./script/run

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

[warp-agent-staging]

This PR was generated with Warp.

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