INST-6 · workSessions (time-series) — active effort across all stages

> Item derived from the fact-checking cycle analysis (**2026-07-03, v2**). Production numbers (namespace `main`), unless noted.

### Finding (evidence)
Separating queue from active effort is only possible today in `draft`, via gaps between saves (<30 min), N=64 — the other stages emit no work signal. Result: **"78% of the clock is queue"** is an aggregate estimate, not a per-stage measurement.

### Action
A **time-series** collection of edit / save / focus events by `reviewTaskId` + `stage` + `userId`; sum into `cycleMetrics.editingActiveH`.

### Impact
"How much human time a fact-check costs" becomes a robust, per-stage number, not an estimate.

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- **Priority:** 🟠 High-leverage · **Effort:** L (large)
- **Note:** engineering flags this as P1-by-leverage. Keep effort **L** and **sequence after INST-2 (#2492)** — it needs the stages instrumented first — but raise it above the other 🟡 items in the value queue.
- **Dependencies:** INST-2 (#2492).
- **Epic:** #2489

Review INST-2 (#2492) first, since this work depends on instrumenting all stages. Then trace the existing draft-stage save-gap measurement and the cycleMetrics path to identify the entry points for edit, save, and focus events keyed by reviewTaskId, stage, and userId. Done means events from every stage aggregate into cycleMetrics.editingActiveH.

Escrito por el modelo de indexación a partir del texto del issue.