feat(coding-agent): add prompt cache miss tracking (#6427)

Detect prompt cache misses per turn by comparing each assistant message's
cache reads against the previous request's prompt tokens (core/cache-stats.ts).
Significant misses emit a warning-colored transcript notice at the turn they
occur, noting idle gaps past the cache TTL and model switches when relevant.

/session gained cache statistics: a compact token/cache breakdown with hit
rate, a $-prefixed cost section with per-model cost breakdown, and the
cumulative cost re-billed due to cache misses.

packages/coding-agent/src/modes/interactive/components/footer.ts

@@ -20,7 +20,7 @@ function sanitizeStatusText(text: string): string {
 /**
  * Format token counts for compact footer display.
  */
-function formatTokens(count: number): string {
+export function formatTokens(count: number): string {
 	if (count < 1000) return count.toString();
 	if (count < 10000) return `${(count / 1000).toFixed(1)}k`;
 	if (count < 1000000) return `${Math.round(count / 1000)}k`;
@@ -137,7 +137,6 @@ export class FooterComponent implements Component {
 		if ((totalCacheRead > 0 || totalCacheWrite > 0) && latestCacheHitRate !== undefined) {
 			statsParts.push(`CH${latestCacheHitRate.toFixed(1)}%`);
 		}
-
 		// Show cost with "(sub)" indicator if using OAuth subscription
 		const usingSubscription = state.model ? this.session.modelRegistry.isUsingOAuth(state.model) : false;
 		if (totalCost || usingSubscription) {