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/tiny/title-client.ts

@@ -39,6 +39,11 @@ export interface TinyTitleDownloadOptions {
 	onProgress?: (event: TinyTitleProgressEvent) => void;
 }
 
+export interface TinyTitleGenerateOptions {
+	signal?: AbortSignal;
+	systemPrompt?: string;
+}
+
 // Cold-starting the worker subprocess from a compiled binary (decompress + module
 // graph load) is slow on contended CI runners — the macos-15-intel release smoke
 // blew past 5s while arm64/linux/win passed. The probe only needs to prove the
@@ -46,6 +51,14 @@ export interface TinyTitleDownloadOptions {
 // generous bound removes the flake without weakening the check.
 const SMOKE_TEST_TIMEOUT_MS = 30_000;
 
+function normalizeTinyTitleGenerateOptions(
+	options: AbortSignal | TinyTitleGenerateOptions | undefined,
+): TinyTitleGenerateOptions {
+	if (!options) return {};
+	if ("aborted" in options && "addEventListener" in options) return { signal: options };
+	return options;
+}
+
 /**
  * Hidden subcommand on the main CLI that boots the tiny-model worker in the
  * spawned subprocess. Kept in sync with the dispatch in `cli.ts`.
@@ -295,9 +308,16 @@ export class TinyTitleClient {
 		return () => this.#progressListeners.delete(listener);
 	}
 
-	async generate(modelKey: string, message: string, signal?: AbortSignal): Promise<string | null> {
+	async generate(modelKey: string, message: string, signal?: AbortSignal): Promise<string | null>;
+	async generate(modelKey: string, message: string, options?: TinyTitleGenerateOptions): Promise<string | null>;
+	async generate(
+		modelKey: string,
+		message: string,
+		optionsOrSignal?: AbortSignal | TinyTitleGenerateOptions,
+	): Promise<string | null> {
+		const options = normalizeTinyTitleGenerateOptions(optionsOrSignal);
 		if (!isTinyTitleLocalModelKey(modelKey)) return null;
-		if (signal?.aborted) return null;
+		if (options.signal?.aborted) return null;
 
 		try {
 			const worker = this.#ensureWorker();
@@ -310,12 +330,15 @@ export class TinyTitleClient {
 				this.#pending.delete(id);
 				pending.resolve(null);
 			};
-			signal?.addEventListener("abort", abort, { once: true });
+			options.signal?.addEventListener("abort", abort, { once: true });
 			try {
-				worker.send({ type: "generate", id, modelKey, message });
+				const request: TinyTitleWorkerInbound = options.systemPrompt
+					? { type: "generate", id, modelKey, message, systemPrompt: options.systemPrompt }
+					: { type: "generate", id, modelKey, message };
+				worker.send(request);
 				return await promise;
 			} finally {
-				signal?.removeEventListener("abort", abort);
+				options.signal?.removeEventListener("abort", abort);
 				this.#pending.delete(id);
 			}
 		} catch (error) {

packages/coding-agent/src/tiny/title-protocol.ts

@@ -29,7 +29,7 @@ export interface TinyTitleProgressEvent {
 
 export type TinyTitleWorkerInbound =
 	| { type: "ping"; id: string }
-	| { type: "generate"; id: string; modelKey: TinyTitleLocalModelKey; message: string }
+	| { type: "generate"; id: string; modelKey: TinyTitleLocalModelKey; message: string; systemPrompt?: string }
 	| { type: "complete"; id: string; modelKey: TinyLocalModelKey; prompt: string; maxTokens?: number }
 	| { type: "download"; id: string; modelKey: TinyLocalModelKey };
 

packages/coding-agent/src/tiny/worker.ts

@@ -436,9 +436,10 @@ async function loadPipeline(
 	return loaded;
 }
 
-function buildPrompt(generator: TextGenerationPipeline, message: string): string {
+function buildPrompt(generator: TextGenerationPipeline, message: string, systemPrompt?: string): string {
+	const selectedSystemPrompt = systemPrompt?.trim() || TINY_TITLE_SYSTEM_PROMPT;
 	const chat = [
-		{ role: "system", content: TINY_TITLE_SYSTEM_PROMPT },
+		{ role: "system", content: selectedSystemPrompt },
 		{ role: "user", content: formatTitleUserMessage(message) },
 	];
 	const chatTemplateOptions = {
@@ -464,9 +465,10 @@ async function generateTitle(
 	requestId: string,
 	modelKey: TinyTitleLocalModelKey,
 	message: string,
+	systemPrompt?: string,
 ): Promise<string | null> {
 	const generator = await loadPipeline(modelKey, transport, requestId);
-	const promptText = buildPrompt(generator, message);
+	const promptText = buildPrompt(generator, message, systemPrompt);
 	const transformers = await loadTransformers(transport, requestId, modelKey);
 	const output = (await generator(promptText, {
 		max_new_tokens: TITLE_MAX_NEW_TOKENS,
@@ -548,7 +550,7 @@ async function handleQueuedRequest(
 			transport.send({ type: "completion", id: request.id, text });
 			return;
 		}
-		const title = await generateTitle(transport, request.id, request.modelKey, request.message);
+		const title = await generateTitle(transport, request.id, request.modelKey, request.message, request.systemPrompt);
 		transport.send({ type: "title", id: request.id, title });
 	} catch (error) {
 		transport.send({ type: "error", id: request.id, error: errorText(error) });