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.

.claude-plugin/marketplace.json

@@ -10,7 +10,7 @@
       "name": "agent-loop",
       "source": "./",
       "description": "Autonomous generator-evaluator agent loop for long-running coding tasks. Plan interactively, then execute with full visibility.",
-      "version": "0.11.0",
+      "version": "0.12.0",
       "author": {
         "name": "Sheldon"
       },