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, 13 insertions, 0 deletions

diff --git a/packages/kernel/src/runtime/events.ts b/packages/kernel/src/runtime/events.ts
index 300e711..b194577 100644
--- a/packages/kernel/src/runtime/events.ts
+++ b/packages/kernel/src/runtime/events.ts
@@ -127,16 +127,29 @@ export function doneEvent(
 	reason: string,
 	durationMs?: number,
 	usage?: Usage,
+	contextSize?: number,
 ): AgentEvent {
+	if (durationMs !== undefined && usage !== undefined && contextSize !== undefined) {
+		return { type: "done", conversationId, turnId, reason, durationMs, usage, contextSize };
+	}
 	if (durationMs !== undefined && usage !== undefined) {
 		return { type: "done", conversationId, turnId, reason, durationMs, usage };
 	}
+	if (durationMs !== undefined && contextSize !== undefined) {
+		return { type: "done", conversationId, turnId, reason, durationMs, contextSize };
+	}
+	if (usage !== undefined && contextSize !== undefined) {
+		return { type: "done", conversationId, turnId, reason, usage, contextSize };
+	}
 	if (durationMs !== undefined) {
 		return { type: "done", conversationId, turnId, reason, durationMs };
 	}
 	if (usage !== undefined) {
 		return { type: "done", conversationId, turnId, reason, usage };
 	}
+	if (contextSize !== undefined) {
+		return { type: "done", conversationId, turnId, reason, contextSize };
+	}
 	return { type: "done", conversationId, turnId, reason };
 }