feat: worktree-based run isolation for parallel loops

Each /agent-loop:run now creates a git worktree for the feature branch
before generating stories. This provides full isolation:

- Multiple loops can run in parallel on different specs in the same project
- Main working directory stays on main, always available
- Each worktree has its own .loop/ state, tmux session, and branch
- Completed runs are archived to main's .loop/archive/ with runs.log

Changes:
- setup.sh: add --init-worktree mode for initializing worktree .loop/
- archive.sh: add archive_from_worktree() for cross-directory archiving
- loop.sh: replace branch checkout with validation (worktree is pre-checked-out)
- agents/planner.md: accept absolute path prefix for worktree .loop/ writes
- skills/run/SKILL.md: full rewrite — worktree creation in Phase 2, launch in
  Phase 3, archive on completion, .active-worktree tracking file
- skills/stories/SKILL.md: worktree-aware, defer to /run for full flow

Bump to 0.12.0.

loop.sh

@@ -181,12 +181,14 @@ if [ -f "$LOOP_DIR/init.sh" ]; then
     bash "$LOOP_DIR/init.sh"
 fi
 
-# Ensure correct git branch
+# Verify we're on the expected branch (worktree should already be on it)
 BRANCH=$(prd_branch_name 2>/dev/null || echo "")
 if [ -n "$BRANCH" ]; then
     CURRENT_BRANCH=$(git rev-parse --abbrev-ref HEAD 2>/dev/null || echo "")
     if [ "$CURRENT_BRANCH" != "$BRANCH" ]; then
-        log "Switching to branch: $BRANCH"
+        log "WARNING: Expected branch '$BRANCH' but on '$CURRENT_BRANCH'"
+        log "If running in a worktree, the branch should already be checked out."
+        log "Attempting to switch..."
         git checkout "$BRANCH" 2>/dev/null || \
             git checkout -b "$BRANCH" "origin/$BRANCH" 2>/dev/null || \
             git checkout -b "$BRANCH"