docs: add Global LLM Worktree Rule to operating rules (#272)

Require mutations only from branches/ worktrees after proving project
root, cwd, branch, stable main-checkout branch, and session worktree
path. Surface in portable skill, runbooks, wiki, operator guide, and
start-issue template.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

skills/llm-project-workflow/templates/start-issue.md

@@ -22,6 +22,12 @@ Steps:
    as lease owner, review the existing PR, hand off, request takeover after
    expiry, or report "work already claimed"). Never open a parallel branch/PR
    unless the old branch is proven abandoned and takeover is recorded.
+0b. Global LLM Worktree Rule — before any mutation, prove and state: project
+   root; cwd; current branch; stable branch for the main checkout (master/main/dev);
+   session-owned worktree path under branches/. If cwd is not inside branches/,
+   STOP (no exceptions — not for docs, tests, small fixes, review fixes, conflicts,
+   or cleanup). Main checkout is control-only: read-only inspect, fetch, create
+   worktrees, stable-branch update after merge, explicit repair.
 1. Identity Checklist: Before claiming work, verify and state:
    - Required identity/profile for this task: author (allowed to push branches / create PRs)
    - Current authenticated identity (from whoami): <username>