Issue builder should wait for non-builder agents to finish before exiting

[markgar]

Problem

The issue builder (builder-N in --role issue mode) exits as soon as it sees no open issues and agents appear idle. But "idle" just means between milestones — the milestone reviewer and validator are still draining their queues. They continue filing bugs and findings after the issue builder has already written its done sentinel and exited. Nobody is left to fix those issues.

In the Stretto test run (20260226-143251), 13 issues were filed after the issue builder shut down, including a build-breaking bug (merge conflict markers committed) and several real findings.

Root cause

Circular dependency in shutdown signals:

• Non-builder agents (reviewer, milestone reviewer, tester, validator) wait for all builder-*.done files before draining and exiting
• The issue builder writes builder-N.done before non-builder agents are finished
• Non-builder agents file issues during their drain phase that nobody picks up

Proposed fix: Split the "done" signal

1. Non-builder agents ignore the issue builder's sentinel. Change the drain trigger so reviewers, milestone reviewer, tester, and validator only wait for milestone builder sentinels (builder-1.done through builder-{N-1}.done), not the issue builder's. This unblocks their drain without the issue builder having to exit first.

2. Non-builder agents write their own done sentinels. After draining, each writes logs/{agent}.done (e.g. validator.done, milestone-reviewer.done, tester.done, reviewer-N.done).

3. Issue builder waits for non-builder done sentinels. Instead of using log-staleness as a proxy for "agents finished," the issue builder explicitly waits for validator.done, milestone-reviewer.done, tester.done, and all reviewer-N.done files. It keeps polling for and fixing open issues while waiting.

4. Issue builder does a final sweep. Once all non-builder sentinels exist and no open bugs/findings remain, the issue builder writes builder-N.done and exits last.

5. Orchestrator waits for all done files. is_builder_done() continues to wait for all builder-*.done files as today — no change needed there.

Shutdown sequence after fix

Milestone builders finish → write builder-{1..M-1}.done
    ↓
Non-builder agents see milestone builders done → drain remaining milestones → file last issues → write {agent}.done
    ↓
Issue builder sees non-builder done files + no open issues → final sweep → write builder-N.done
    ↓
Orchestrator sees all builder-*.done → exit

Files likely affected

• src/agentic_dev/sentinel.py — is_builder_done() needs a variant that excludes the issue builder; new is_agent_done() / write_agent_done() helpers
• src/agentic_dev/builder.py — issue builder shutdown loop: wait for non-builder sentinels instead of log-staleness
• src/agentic_dev/watcher.py — reviewer drain trigger: only check milestone builder sentinels
• src/agentic_dev/milestone_reviewer.py — same drain trigger change; write done sentinel after drain
• src/agentic_dev/tester.py — same
• src/agentic_dev/validator.py — same
• src/agentic_dev/orchestrator.py — clear non-builder sentinels at startup alongside builder sentinels