[WIP] Fix approval gate timeout handling in pipelines - #5

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

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This section details on the original issue you should resolve

<issue_title>Approval gate timeout leaves pipeline in unrecoverable rejected-at-diff state</issue_title>
<issue_description>## Bug Report

Summary

When the diff approval gate times out (default 120s), the pipeline is permanently stopped with rejected-at-diff status. There is no way to continue, retry, or resume the task.

Steps to Reproduce

1. Run devloop run without --auto
2. When the diff gate appears, let it time out (120s)
3. Pipeline prints: Approval timed out after 120s - rejecting
4. Pipeline prints: Diff rejected at approval gate - pipeline stopped
5. Open devloop view TASK-ID - status shows rejected-at-diff with no recovery path

Observed Behavior

After timeout:

• Status file set to rejected-at-diff
• Live View shows phases done but status = rejected-at-diff
• devloop resume re-runs the entire worker phase (wrong - worker already completed)
• No hint printed about how to recover

Expected Behavior

A timeout at a gate is different from an active rejection. Expected:

• devloop resume TASK-ID should detect gate-stall and re-present only the gate
• devloop resume TASK-ID --approve-diff to force-approve and proceed to review
• Recovery hint printed after timeout: Run devloop resume TASK-ID to retry the gate
• New status values: timed-out-at-plan, timed-out-at-diff (distinct from rejected)

Suggested Fix

1. Add timed-out-at-diff status distinct from rejected-at-diff in session status file
2. cmd_resume() should detect gate-stall status and re-present the gate, not re-run worker
3. Consider per-gate timeout config: DEVLOOP_PLAN_GATE_TIMEOUT, DEVLOOP_DIFF_GATE_TIMEOUT
4. Print recovery hint immediately after timeout

Environment

• DevLoop: v5.1.0
• Gate: diff gate
• DEVLOOP_APPROVAL_TIMEOUT: 120 (default)

Related Code

• devloop.sh _approval_gate() - gate resolver and timeout
• devloop.sh cmd_resume() - needs to handle gate-stall status
• .devloop/sessions/TASK-ID/status - needs new timed-out states
  </issue_description>

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