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, 7 insertions, 0 deletions

diff --git a/packages/kernel/src/runtime/run-turn.ts b/packages/kernel/src/runtime/run-turn.ts
index b50e8ee..bf57854 100644
--- a/packages/kernel/src/runtime/run-turn.ts
+++ b/packages/kernel/src/runtime/run-turn.ts
@@ -449,6 +449,7 @@ export async function runTurn(input: RunTurnInput): Promise<RunTurnResult> {
 	const messages: ChatMessage[] = [...input.messages];
 	const resultMessages: ChatMessage[] = [];
 	let totalUsage = zeroUsage();
+	let lastStepUsage: Usage | undefined;
 	let finishReason = "stop";
 
 	const toolMap = new Map<string, ToolContract>();
@@ -513,6 +514,7 @@ export async function runTurn(input: RunTurnInput): Promise<RunTurnResult> {
 			});
 
 			totalUsage = addUsage(totalUsage, stepResult.usage);
+			lastStepUsage = stepResult.usage;
 
 			if (stepResult.assistantMessage !== undefined) {
 				messages.push(stepResult.assistantMessage);
@@ -571,6 +573,10 @@ export async function runTurn(input: RunTurnInput): Promise<RunTurnResult> {
 		totalUsage.outputTokens > 0 ||
 		totalUsage.cacheReadTokens !== undefined ||
 		totalUsage.cacheWriteTokens !== undefined;
+	const contextSize =
+		hasUsage && lastStepUsage !== undefined
+			? lastStepUsage.inputTokens + lastStepUsage.outputTokens
+			: undefined;
 	input.emit(
 		doneEvent(
 			conversationId,
@@ -578,6 +584,7 @@ export async function runTurn(input: RunTurnInput): Promise<RunTurnResult> {
 			finishReason,
 			turnDurationMs,
 			hasUsage ? totalUsage : undefined,
+			contextSize,
 		),
 	);