feat: auto-name sessions from first prompt

[shenlvkang-collab]

Problem

New sessions are initially named with generic project/sequence labels, so it is difficult to identify their purpose in the session manager.

Solution

• Derive a bounded local title from the first submitted prompt.
• Ignore slash commands and terminal control sequences.
• Persist whether a name is auto or manual.
• Never overwrite an explicit rename.
• Update mux metadata and broadcast the new name immediately.

No prompt text is sent to an additional model or written to logs.

Verification

• typecheck
• lint
• build
• frontend syntax check
• focused input/listener tests: 23/23 passed

The implementation was also adapted and validated against the older Codeman deployment on silverdou.