feat(coding-agent): add configurable title system prompt for sessions

- add discovery of `TITLE_SYSTEM.md` and pass it through interactive startup context
- route custom title prompts to online and local tiny title generators via protocol
- update session-title docs and changelog with override behavior
- add tests for prompt discovery, forwarding, and fallback to bundled title prompts

packages/coding-agent/src/modes/controllers/input-controller.ts

@@ -467,6 +467,7 @@ export class InputController {
 					this.ctx.session.sessionId,
 					this.ctx.session.model,
 					provider => this.ctx.session.agent.metadataForProvider(provider),
+					this.ctx.titleSystemPrompt,
 				)
 					.then(async title => {
 						// Re-check: a concurrent attempt for an earlier message may have

packages/coding-agent/src/modes/interactive-mode.ts

@@ -260,6 +260,7 @@ export class InteractiveMode implements InteractiveModeContext {
 	keybindings: KeybindingsManager;
 	agent: Agent;
 	historyStorage?: HistoryStorage;
+	titleSystemPrompt?: string;
 
 	ui: TUI;
 	chatContainer: TranscriptContainer;
@@ -382,6 +383,7 @@ export class InteractiveMode implements InteractiveModeContext {
 		lspServers: LspStartupServerInfo[] | undefined = undefined,
 		mcpManager?: import("../mcp").MCPManager,
 		eventBus?: EventBus,
+		titleSystemPrompt?: string,
 	) {
 		this.session = session;
 		this.sessionManager = session.sessionManager;
@@ -394,6 +396,7 @@ export class InteractiveMode implements InteractiveModeContext {
 		this.lspServers = lspServers;
 		this.mcpManager = mcpManager;
 		this.#eventBus = eventBus;
+		this.titleSystemPrompt = titleSystemPrompt;
 		if (eventBus) {
 			this.#eventBusUnsubscribers.push(
 				eventBus.on(LSP_STARTUP_EVENT_CHANNEL, data => {

packages/coding-agent/src/modes/types.ts

@@ -99,6 +99,7 @@ export interface InteractiveModeContext {
 	historyStorage?: HistoryStorage;
 	mcpManager?: MCPManager;
 	lspServers?: LspStartupServerInfo[];
+	titleSystemPrompt?: string;
 
 	// State
 	isInitialized: boolean;