Restore student PR comments

[morganmcg1]

What changed

• Add a student-only post_assignment_comment tool for meaningful progress updates, questions, blockers, evidence, and review replies on the exact assigned PR.
• Bind the student identity in the runtime and validate the open PR, assignment ID, revision, current head SHA, student label, and WIP-or-review routing before and after posting.
• Make comments immutable and idempotent through typed markers and stable comment_id values, without pushing or changing branches, draft state, or labels.
• Deliver trusted student comments to the advisor exactly once. Exact concurrent duplicates collapse to one semantic event, and immutable comments are edge-triggered rather than repeated on reminder cadence.
• Document the restored workflow for students and harness operators.

Why

Students could publish a terminal experiment result but had no typed way to reply or report interim status on their assignment PR. Their internal responses therefore stayed inside the conversation, leaving advisors and humans with a misleadingly silent GitHub thread.

Validation

• 919 passed, 6 skipped from the complete test suite.
• 178 passed in the focused controller, GitHub workflow, mailbox, tool-schema, and marker suite.
• git diff --cached --check passes.