Auto-generate descriptive titles for agent tabs

[uzayer]

Give new agent-chat tabs an automatic, concise title derived from the conversation, similar to Cursor.

Acceptance criteria:

• New tabs begin with a neutral temporary title.
• After enough user/assistant context exists, generate a short task-oriented title automatically.
• Do not overwrite a user-renamed tab.
• Title generation is debounced/deduplicated and must not interrupt chat streaming or add visible noise.