fix(fleet): derive pane idle from window activity fallback

docs/scratchpads/h1b-pane-idle-signal.md

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+# H1b — tmux pane idle signal wiring
+
+## Objective
+
+Feed `classifyReadiness()` a real idle signal on tmux 3.4 by deriving `idleSeconds` from the first available tmux timestamp source: pane activity, then window activity, then session activity.
+
+## Scope
+
+- `packages/mosaic/src/commands/fleet.ts`
+  - Extend `buildTmuxListPanesCommand()` format to include `#{window_activity}` and `#{session_activity}` after the existing fields.
+  - Update `parseTmuxListPanes()` to choose the first non-empty finite positive timestamp and clamp future idle values to 0.
+- `packages/mosaic/src/commands/fleet.spec.ts`
+  - Cover pane/window/session activity parsing behavior, empty-field index alignment, null idle, future clamping, math correctness, and exact tmux format.
+
+## Out of Scope
+
+- No changes to `classifyReadiness()`, thresholds, `AgentPsRow`, or `fleet ps` rendering.
+- No merge by worker; orchestrator routes review/merge.
+- Workers do not modify `docs/TASKS.md`.
+
+## PRD Alignment
+
+Aligned with `docs/fleet/PRD.md` FR-1 and acceptance criteria for truthful `mosaic fleet ps` pane/pid/idle observability.
+
+## Plan
+
+1. Sync branch from latest `origin/main` and install dependencies with required pnpm env.
+2. Add/confirm reproducer tests for tmux 3.4 empty `pane_activity` and new fallback behavior.
+3. Implement the focused parser/format change only.
+4. Run required build, baseline gates, fleet vitest, and independent review.
+5. Run pre-push queue guard, push branch, and open PR to `main` with Mosaic wrapper.
+
+## Progress
+
+- 2026-06-24: Branch `fix/fleet-pane-idle-activity` created from `origin/main` @ `ec8dd7c` after fetching.
+- 2026-06-24: Session-start generated local `.mosaic/orchestrator/*` changes on the previous release branch; stashed as `coder1 session-start state before H1b` to keep this branch clean.
+- 2026-06-24: Added TDD coverage for the tmux 3.4 production case (`pane_activity` empty, `window_activity` populated), exact new list-panes format, null/future/multiple-source behavior.
+- 2026-06-24: Implemented parser fallback without changing readiness classifier thresholds or render shape.
+
+## Verification Evidence
+
+- `pnpm install --store-dir "$HOME/.pnpm-store"` — pass.
+- Reproducer before implementation: `pnpm --filter @mosaicstack/mosaic exec vitest run src/commands/fleet.spec.ts` — failed as expected (old format, no fallback, negative future idle).
+- `npx turbo build --filter=@mosaicstack/mosaic^...` — pass, 12/12 tasks successful.
+- `pnpm typecheck` — pass, 41/41 tasks successful.
+- `pnpm lint` — pass, 23/23 tasks successful.
+- `pnpm format:check` — pass, all matched files use Prettier style.
+- `pnpm --filter @mosaicstack/mosaic exec vitest run src/commands/fleet.spec.ts` — pass, 176 tests.
+- `~/.config/mosaic/tools/codex/codex-code-review.sh --uncommitted` — approve, 0 findings (reviewed supplied diff; sandbox file-inspection limitation noted by tool).
+
+## Risks / Blockers
+
+- No current blocker.