feat(metrics): consume contextSize — current context-usage readout

Backend context-size handoff: re-pin [email protected] / [email protected]
(+ re-mirror .dispatch reference snapshots). Thread the optional contextSize
through core/metrics (done fold + durable + selectCurrentContextSize: latest
turn's defined value, undefined=>unknown never 0, durable-wins-over-live).
Chat store exposes currentContextSize; ContextSizeBadge renders
"N tokens in context" / "context size unknown" above the composer.
GLOSSARY: add context size / context window. 533 tests green.

1 files changed, 11 insertions, 0 deletions

diff --git a/src/core/metrics/format.ts b/src/core/metrics/format.ts
index ee8db60..d8dd2cc 100644
--- a/src/core/metrics/format.ts
+++ b/src/core/metrics/format.ts
@@ -17,6 +17,17 @@ function formatTps(tps: number | null): string | null {
 	return `${Math.round(tps)} tok/s`;
 }
 
+/**
+ * Format the current context size for display. A defined count renders as
+ * `"<n> tokens in context"` (thousands-separated); `undefined` ("unknown" — no
+ * per-step usage reported yet) renders the placeholder `"context size unknown"`.
+ * Never renders `0` for the unknown case.
+ */
+export function formatContextSize(n: number | undefined): string {
+	if (n === undefined) return "context size unknown";
+	return `${formatTokens(n)} tokens in context`;
+}
+
 /** Compute tokens-per-second. Returns null when elapsed time is absent or zero. */
 export function computeTps(outputTokens: number, elapsedMs: number | undefined): number | null {
 	if (elapsedMs === undefined || elapsedMs <= 0) return null;