/plan-pipeline:execute: Execution to merged PR

Execute the validated plan in an isolated worktree. Spawn per-task agents, verify quality, create and merge the PR. Handles everything through to cleanup.

Run /clear before this command.

Prerequisite

A validated plan must exist at docs/plans/plan-{name}.md with all issues resolved (output of /plan-pipeline:validate).

Step 1: Worktree setup

Create an isolated git worktree: