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/core/settings-manager.ts

@@ -92,6 +92,7 @@ export interface Settings {
 	branchSummary?: BranchSummarySettings;
 	retry?: RetrySettings;
 	hideThinkingBlock?: boolean;
+	showCacheMissNotices?: boolean; // default: false - show transcript notices for significant prompt-cache misses
 	externalEditor?: string; // Command for Ctrl+G external editor; takes precedence over VISUAL/EDITOR
 	shellPath?: string; // Custom shell path (e.g., for Cygwin users on Windows)
 	quietStartup?: boolean;
@@ -845,6 +846,10 @@ export class SettingsManager {
 		return this.settings.hideThinkingBlock ?? false;
 	}
 
+	getShowCacheMissNotices(): boolean {
+		return this.settings.showCacheMissNotices ?? false;
+	}
+
 	getExternalEditorCommand(): string | undefined {
 		const configuredEditor = this.settings.externalEditor;
 		if (typeof configuredEditor === "string" && configuredEditor.trim() !== "") {
@@ -863,6 +868,12 @@ export class SettingsManager {
 		this.save();
 	}
 
+	setShowCacheMissNotices(show: boolean): void {
+		this.globalSettings.showCacheMissNotices = show;
+		this.markModified("showCacheMissNotices");
+		this.save();
+	}
+
 	getShellPath(): string | undefined {
 		return this.settings.shellPath;
 	}