feat(cache-warming): per-conversation prompt-cache warming + warm() service

Backend-driven warming targeting whatever provider a conversation uses (incl. the
external Claude provider-anthropic). Core engine + on/off + last-cache-% done;
interval-as-view-control pending a ui-contract NumberField (surface-system gap).

Mechanism:
- kernel: expose HostAPI.emit (typed bus event emit; counterpart of on)
- session-orchestrator: turnStarted/turnSettled event hooks (conversationId/cwd/model);
  warm() service (cacheWarmHandle) reusing the real-turn assembly (byte-identical prefix,
  provider-agnostic), refuses mid-turn, never persists/emits, returns Usage
- cache-warming (new ext): per-conversation timers (arm on settle, cancel on start,
  in-flight invalidation), calls warm(), pct=round(clamp(cacheRead/input,0,1)*100),
  persists {enabled,intervalMs} (default on/240s), registers a controls surface
- host-bin: register cache-warming; transport-http: HostAPI stub +emit (fan-out)

Honors old-code invariants. 760 vitest + 109 bun = 869 tests; tsc -b EXIT 0; biome clean.

1 files changed, 26 insertions, 1 deletions

diff --git a/tasks.md b/tasks.md
index 7ccfbcd..0bde08f 100644
--- a/tasks.md
+++ b/tasks.md
@@ -5,7 +5,7 @@
 > Keep this lean and current; do not let it re-accrete a step-by-step changelog.
 
 ## Status (current)
-`tsc -b` EXIT 0 · biome clean · **734 vitest + 109 bun = 843 tests**.
+`tsc -b` EXIT 0 · biome clean · **760 vitest + 109 bun = 869 tests**.
 
 Built and verified live (full-fidelity: every feature is a manifest-loaded
 extension through the host):
@@ -140,6 +140,31 @@ no summary but still loadable. Glossary: added `skill`, `skill summary`, `tools
   `tsc -b` EXIT 0, biome clean. (End-to-end load_skill via a real LLM turn not yet exercised —
   unit/integration tests cover the filter rewrite + live read.)
 
+## Cache warming (core DONE; control surface PARTIAL)
+User-gated calls: target the external **Claude** provider (`../claude` provider-anthropic, loaded via
+`DISPATCH_EXTERNAL_EXTENSIONS`); warm-assembly lives in **session-orchestrator** (`warm()` reuses the
+real turn's assembly → byte-identical prefix, provider-agnostic); **surface system** for controls;
+**per-conversation** controls; interval default 4 min, free value. Old-code invariants honored
+(primary-model/full-prefix via reuse; refuse mid-turn; never persist/emit; in-flight invalidation;
+arm-on-settle/cancel-on-start; `pct = round(clamp(cacheRead/input,0,1)*100)`).
+- **Mechanism (2nd use of bus hooks; first event-hook emit):**
+  - [x] **kernel** — exposed `HostAPI.emit` (delegates to bus.emit), counterpart of `on`.
+  - [x] **session-orchestrator** — `turnStarted`/`turnSettled` event hooks (carry conversationId/cwd/
+    modelName) emitted per turn; `warm()` service (`cacheWarmHandle`) reusing assembly, refusing
+    mid-turn, never persisting/emitting; returns Usage.
+  - [x] **cache-warming** (new ext) — per-conversation timers (arm/cancel/in-flight token),
+    calls `warm()`, computes `lastPct`, persists `{enabled,intervalMs}` (default on/240s) in
+    host.storage; registers a controls Surface. 19 tests.
+  - [x] **host-bin** — registered cache-warming; **transport-http** HostAPI stub fixed for `emit`.
+- **Live-verified:** full-graph `tsc -b` EXIT 0, biome clean (boot smoke + live Claude warm pending
+  a restart with the cache-warming ext loaded).
+- **OPEN — surface-system limits (CR from cache-warming):** the surface system has (a) NO
+  per-conversation context (surface reflects most-recently-active conversation; invoke carries
+  conversationId), and (b) NO numeric-input field kind, so the **interval ("set time to refresh")
+  control is not yet a view input** — only the on/off toggle + last-cache-% stat render. Honoring
+  per-conversation controls + free-value interval needs a `NumberField` in `ui-contract` +
+  per-conversation surface scoping (+ FE courier). Decision pending.
+
 ## Open items
 - **`prefix.fingerprint` / `warm|real` cache-bust attributes (deferred):** decoupled
   from dedup by the content-addressed decision; also gated on cache-warming being