feat(#1680): ADR-1239 Phase C-1 — hook-bus + stateIO seams [AC4] (#1805)

* feat(#1680): ADR-1239 Phase C-1 — hook-bus + stateIO seams [AC4]

Phase 3 slice 4 (AC4, final #1680 slice). The last two adapter seams behind
the negotiated hookBus/stateIO axes:

- src/hook-bus.cts: createHookBus({bus}, {hostEmit?}) -> host/engine/none.
  engine = in-process pub/sub (handler errors isolated); host = host-owned,
  fail-closed emit until a host emitter is bound; none = silent no-op (degrade
  to rule-text). PORTABLE_EVENT_FLOOR = SessionStart/PreToolUse/PostToolUse/
  Stop/SessionEnd (the claude dialect all hook hosts share).
- src/state-io.cts: createStateIO({io}, {backend?}) -> filesystem (today's
  behavior — straight fs) / sandboxed-storage / session-log-append (fail-closed
  seams until a host backend is bound).

Proactive CI gates: ADR-457 ignores + INVENTORY-MANIFEST entries for both new
.cjs; injection-scan 'act as' substring audit; unused-import check. All clean
locally (11 tests + security 15/15 + inventory + eslint 0 problems).

Phase 3 (#1680) seam layer now complete. Concrete host binding -> Phase 5
(#1682, D15/D18).

* chore(changeset): add Changed fragment for hook-bus + stateIO seams (#1680)

.changeset/plucky-moles-sing.md

@@ -0,0 +1,7 @@
+---
+type: Changed
+pr: 1805
+---
+**Internal: hook-bus + stateIO adapter seams** — `createHookBus({bus})` (new `src/hook-bus.cts`, `host`/`engine`/`none` — engine is in-process pub/sub, host fail-closed, none silent) + `createStateIO({io})` (new `src/state-io.cts`, `filesystem`/`sandboxed-storage`/`session-log-append` — filesystem delegates to fs, the rest are fail-closed seams) (ADR-1239 Phase C-1 / #1680 AC4). Completes the Phase 3 adapter seam layer; concrete host binding is Phase 5. No user-facing change.
+
+<!-- docs-exempt: internal adapter seams; no user-facing command/flag/config/schema/doc surface -->