pr-flow: always start work in a git worktree #2661

Problem

pr-flow step 2 (Branch) currently treats a worktree as optional:

Working in a git worktree is fine and often preferable.

In practice this repo is worked by several concurrent agent sessions on one machine (the premise behind #2338, #2339, #2473). When a session cuts its feature branch in the primary checkout, it switches the branch out from under any other session using that checkout, and uncommitted work, build output and node_modules state from one task leak into another.

Proposal

Make "start work in a worktree" the default in pr-flow, not an aside:

• Step 2 (Branch): create the feature branch as a new worktree cut from the target base, e.g.

  git fetch origin
  git worktree add ../inspector-<issue> -b v2/fix/<issue>-<slug> origin/v2/main
  cd ../inspector-<issue> && npm install

  (or the harness's EnterWorktree where available), and do all subsequent work there.
• Keep the existing ⚠️ that a worktree needs a real npm install, not a symlinked node_modules — promote it into the recipe itself rather than a trailing note.
• Closing out (after merge): add removing the worktree (git worktree remove) and deleting the local branch.
• Update the one-line rule in AGENTS.md → Issue-driven Work Style ("When work begins, assign the issue to yourself, create a feature branch…") to say the branch is created in a worktree, so the rule and the recipe agree.
• Cross-check local-dev / pre-push-gate, which already document worktree hazards, so they point at the same recipe rather than describing a second one.

Acceptance

• pr-flow step 2 creates the branch in a new worktree by default, with the npm install step inline.
• Close-out step removes the worktree.
• AGENTS.md rule updated to match.
• npm run verify:skills passes; re-run skills:eval for pr-flow if its description changes.