Start a new thread from outside the app: t3code://new?project=…&prompt=…

[jpsirois]

Problem
I keep my work todos in another app (Oak). Its todos come from meeting recordings and are filed by project. When I want an agent to do a todo, I copy the text, switch to T3 Code, pick the project, start a thread, and paste. No external app can start a T3 Code thread: the desktop app has no URL route or CLI for it, and the t3_thread_launch tool works only inside an agent session.

Proposal
A URL that opens the new-thread composer with the fields filled in, for example:
t3code://new?project=<workspace root or project id>&prompt=<url-encoded text>

• project: the project to start in. If no project matches, show the project picker.
• prompt: the composer text. The user still presses send, so a link alone cannot run an agent.
• Optional later: worktree=new|root, branch=<name>.

Open questions

• Should the route take a project ID, a folder path, or both?
• Fill the composer only, or also send? I suggest fill only, for safety.

Related: #14287 and #6834 (open an existing thread), #7051 (open a folder from a CLI).