feat(metrics): expose current context size to the frontend

contextSize = the turn's FINAL step inputTokens+outputTokens (true context
occupancy; NOT the aggregate usage, which sums per-step prompts and overcounts
multi-step turns). Stamped on both the live done event (kernel) and persisted
TurnMetrics (session-orchestrator); a client reads the latest turn's value.

- @dispatch/wire 0.4.0->0.5.0: optional contextSize on TurnDoneEvent + TurnMetrics
- @dispatch/transport-contract 0.5.0->0.6.0 (re-export only)
- glossary: context size (reserve 'context window' for the model limit, later)
- FE courier: frontend-context-size-handoff.md

881 vitest pass; tsc -b EXIT 0; biome clean.

1 files changed, 21 insertions, 1 deletions

diff --git a/tasks.md b/tasks.md
index e756612..85a3e79 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 · **865 vitest + 135 bun = 1000 tests**.
+`tsc -b` EXIT 0 · biome clean · **881 vitest + 135 bun = 1016 tests**.
 
 Built and verified live (full-fidelity: every feature is a manifest-loaded
 extension through the host):
@@ -231,6 +231,26 @@ workspace root, working directory.
   finished it directly; also fixed a real design bug the agent missed: the manager
   read config statically instead of per-cwd (would have broken Roblox).
 
+## Context size — current context-window usage (DONE)
+User-gated decisions: term = **context size** (current usage; reserve "context window" for the
+model's max LIMIT, a later feature); definition = the turn's **FINAL step `inputTokens +
+outputTokens`** (NOT the aggregate `usage`, which sums per-step prompts and overcounts a
+multi-step turn); delivery = a backend-computed field on BOTH the live `done` event and the
+persisted `TurnMetrics`.
+- [x] **Contract (orchestrator):** optional `contextSize?: number` added to `TurnDoneEvent` +
+  `TurnMetrics` in `@dispatch/wire` (`0.4.0→0.5.0`); `@dispatch/transport-contract`
+  `0.5.0→0.6.0` (re-exports both — no other change). Glossary: added **context size**.
+- [x] **Wave (parallel, disjoint pkgs):**
+  - [x] **kernel** — `run-turn.ts` tracks the last step's `Usage`; `doneEvent()` stamps
+    `done.contextSize = lastStep.input + lastStep.output` (omitted when no usage). +3 tests.
+  - [x] **session-orchestrator** — `metrics.ts build()` stamps `TurnMetrics.contextSize` from
+    the final per-step metrics (same definition; equals the live value). +5 tests.
+- [x] Verified: `tsc -b` EXIT 0, biome clean, 881 vitest pass; both owners stayed in-lane.
+  `conversation-store` (JSON passthrough) + `transport-http` (forwards/serves) unchanged.
+- [x] **FE courier handoff:** `frontend-context-size-handoff.md` (user couriers to
+  `../dispatch-web`). Not yet exercised end-to-end against a live LLM (unit tests cover both
+  producers); optional live-verify deferred.
+
 ## Open items
 - **`prefix.fingerprint` / `warm|real` cache-bust attributes (deferred):** decoupled
   from dedup by the content-addressed decision; also gated on cache-warming being