Add per-launch agent, model, and yolo overrides to open and new

[jongio]

Problem

dispatch open and dispatch new always read the agent, model, and yolo settings from the saved config. To resume one session with a different model, or start a one-off session under a different agent, you have to change the config first and remember to change it back. That is friction for a quick experiment.

Proposed solution

Add --agent, --model, and --yolo flags to dispatch open and dispatch new. When present, a flag overrides the config value for that single launch only. Nothing is written back to config. This follows the usual precedence where an explicit flag wins over the stored setting.

--yolo is a boolean flag (--yolo or --yolo=false). --agent and --model take a value.

Acceptance criteria

• dispatch open <id> --model <name> resumes the session with the given model, leaving config untouched.
• dispatch new --agent <name> starts a new session under the given agent.
• --yolo and --yolo=false toggle yolo mode for that launch.
• The overrides apply to every launch mode (inplace, tab, window, pane) and to open --print.
• Invalid usage (missing value for --agent/--model) returns a clear error.
• Shell completion flag lists and the README document the new flags.
• Tests cover parsing, precedence over config, and --print output.

Complexity

S

Priority

High