Add a t3 thread settle CLI command

[samohtnt]

What: a t3 thread settle <threadId> subcommand, and unsettle to match. Today t3 thread has only list, start, send and show.

Why: I run an orchestrator that drives a headless T3 server with the t3 CLI. It starts one thread per build or review run, polls it with thread show, and reads the result. Once it accepts a run's result, the thread is finished, but the orchestrator can't settle it, so it stays in my inbox until auto-settle runs. The shortest auto-settle setting is 1 day.

The server already has the thread.settle command (MCP t3_thread_organize), and the CLI already calls orchestrator tools for thread list. Exposing settle the same way would let any CLI-driven automation clean up after itself without holding a WebSocket token or relying on internal RPC.

Nice to have: --json output, and an exit code that tells "already settled" apart from "no such thread".