feat(orchestrator): soft occupancy threshold requests handoff early (S11 partial)

Coordinator.Soft (default 0.55, ORCHESTRA_OCCUPANCY_SOFT) makes rotate()
and TurnDecision request a handoff advisory-only once occupancy crosses
the soft threshold, well before Hard forces a release/rotation.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01W1rkJ2hBMybnJctPbcy4tT

AUDIT.md

@@ -882,6 +882,34 @@ and `TaskAmended` in `store.apply` now populate/update all four fields
 as amendable. Covered by `TestTaskAmendedAppliesAllFields`
 (`internal/store/store_test.go`).
 
+## S11 — partial fix, 2026-07-27 (soft threshold)
+
+Of S11's four missing pieces (soft ~55% threshold, milestone rotation, thrash
+detection, agent-initiated `ROTATE`), only the first is landed here.
+
+`Coordinator` gained a `Soft float64` field (default 0.55 via `soft()` when
+unset, configurable through `ORCHESTRA_OCCUPANCY_SOFT`). Both `rotate()` (the
+periodic ticker) and `TurnDecision` (the synchronous per-turn path) now check
+occupancy against `Soft` before `Hard`: at or above soft but below hard, they
+call the adapter's `HandoffRequester.RequestHandoff` once (same
+"ask, don't invent" convention as the hard-threshold path,
+`Session.HandoffRequested` guarding re-prompts) and — for `TurnDecision` —
+return `prepare_handoff` without requiring a turn boundary, since this is
+advisory: the agent keeps working, the task stays leased. Only once occupancy
+clears `Hard` does the existing boundary-check/release path run. Covered by
+`TestTurnDecision/"prepare_handoff at soft threshold, below hard, without a
+turn boundary"` (internal/orchestrator/rotation_test.go), asserting a
+handoff request fires and the task remains `StateLeased`.
+
+**Still open from S11:** milestone rotation, thrash detection (N failed test
+runs / same file M times / identical tool calls as a circuit breaker with
+`reason=thrash` and populated `dead_ends`), and agent-initiated `ROTATE`. All
+three need either transcript/tool-call introspection this repo doesn't yet
+have a source for, or an explicit in-band signal from the agent — bigger than
+a threshold comparison, not attempted this pass.
+
+`go build ./...`, `go vet ./...`, `go test ./...` all pass.
+
 ### Design consequences (not yet implemented)
 
 1. **Percentages are a level, not a delta.**