fix: resolve stale Amber Session check runs stuck in_progress

[ambient-code]

Summary

• Root cause: The batch-pr-fixer job created in_progress check runs when dispatching sessions but never updated them to completed when the session finished. Healthy PRs were skipped entirely without resolving their stale checks. Additionally, the handle-comment job always created new check runs instead of updating existing ones.
• Adds update_check_run, resolve_stale_checks, and get_existing_check_run_id functions to the batch Python script
• Resolves stale in_progress checks for healthy/skipped PRs on each batch cycle
• Updates existing in_progress checks instead of creating duplicates (both handle-comment and batch jobs)
• Polls pending sessions after dispatch (up to 30 min) and updates checks on completion
• In the handle-comment job, finds and updates existing in_progress checks via PATCH instead of creating new ones via POST

Test plan

• Trigger the batch job via workflow_dispatch and verify stale checks on healthy PRs are completed
• Comment @ambient-code on a PR and verify the existing in_progress check is updated (not duplicated)
• Verify a new session dispatched by the batch job has its check updated to success/failure after completion

Closes #1253

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🤖 Ambient Session

[ambient-code]

This fixes the Amber Session check run staying in_progress indefinitely (#1253).

Root cause: The batch-pr-fixer job created in_progress check runs when dispatching sessions but had no mechanism to update them to completed when the session finished. Additionally, healthy PRs were skipped without resolving their stale checks.

What changed:

1. Added update_check_run() — PATCHes an existing check run with the session's final status
2. Added resolve_stale_checks() — finds all in_progress "Amber Session" checks on a PR's head commit and completes them based on the session phase
3. Healthy/skipped PRs now get their stale checks resolved on each batch cycle
4. After dispatching new sessions, polls for completion (up to 30 min) and updates checks
5. The handle-comment job now updates existing in_progress checks instead of creating duplicates

🤖 Session

[Gkrumbach07]

@ambient-code

[ambient-code]

Resolved merge conflicts with main and applied the proper fix for stale check runs:

1. handle-comment job: Now finds existing in_progress "Amber Session" check runs via the GitHub API and updates them with PATCH instead of always creating new ones with POST
2. batch-pr-fixer job: Added update_check_run(), resolve_stale_checks(), and get_existing_check_run_id() helpers. Healthy/skipped PRs now get their stale checks resolved each cycle. After dispatching sessions, polls for completion (up to 30 min) and updates checks accordingly.

The previous session had incorrectly deleted the entire workflow file instead of modifying it — this commit restores it from main and applies the targeted fix.

🤖 Session