fix: record metrics for ambient gate declines

[time-attack]

Addresses the missing ambient-gate metrics in #609, reported by @ianTPE from a running QM deployment. This intentionally does not close the broader issue.

When the ambient reply gate chose silence or an emoji reaction, the orchestrator returned before writing a turn metric. Record one metric at that existing decision point, with the actual status, session, scope, optional run ID, and detection duration.

No answer harness runs on this path, so totalMs is zero. Admission-to-decision time is recorded in the existing ingressMs field; detectMs remains its detection-phase subset. This preserves the meaning of completed-turn timing fields instead of mixing two time bases.

Scope

• 10 production lines in the existing decline branch.
• No new metric fields, schema changes, public status changes, dashboard redesign, or changes to the gate's decision.
• Other early-return paths and the broader delivered-vs-silent ambiguity in A turn's outcome isn't recorded faithfully: silent means two opposite things, and suppressed turns are absent from turn_metrics #609 remain separate work.

Validation

• Silent and reaction regressions failed with zero rows before the fix and pass afterward.
• Tests assert exactly one main metric, correct run/session/scope, zero harness duration, and ingress duration covering detection. A normal answered-turn control is unchanged.
• Orchestrator suite: 150 passed.
• Admin metrics, metrics sink, and turn-metrics route suites: 11 passed.
• Core typecheck, full ESLint, Oxlint, formatting, and Knip passed.
• Independent review caught the initial timing-basis mismatch; it was corrected and re-reviewed successfully.

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