/revise agent should read PR comments for revision context

[norrietaylor]

Request

The /revise agent should be smart enough to read the comments on the PR and use them as revision context, rather than relying on the user to restate intent.

Rationale

Reviewers and authors leave their feedback directly in PR comments. When a user triggers /revise, the agent should pull in the PR's comment thread (review comments and discussion) and incorporate that feedback into the spec revision automatically.

Acceptance

• On /revise, the agent fetches the PR comments.
• Comment feedback is treated as input that drives the revision.
• The user does not have to manually paste or re-summarize feedback already present in the thread.

_{Filed from Slack conversation with Karl Godard.}