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