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]>

docs/llm-workflow-runbooks.md

@@ -257,6 +257,34 @@ Gitea-Tools lease gates: `gitea_lock_issue` (fail-closed before author
 mutations), `status:in-progress`, and claim comments. Full portable wording:
 [`skills/llm-project-workflow/SKILL.md`](../skills/llm-project-workflow/SKILL.md).
 
+## Global LLM Worktree Rule
+
+The main project checkout is a stable control checkout. It must stay on the
+configured stable branch: `master`, `main`, or `dev`.
+
+All LLM task work must happen inside the project's `branches/` directory.
+
+Before any mutation, prove:
+
+1. current project root
+2. current working directory
+3. current branch
+4. stable branch for the main checkout
+5. session-owned worktree path under `branches/`
+
+If `cwd` is not inside `branches/`, stop. Do not edit, create, delete, format,
+test-write, commit, merge, rebase, checkout task branches, resolve conflicts,
+or run cleanup.
+
+There are no exceptions for small fixes, docs, tests, cleanup, PR review fixes,
+conflict resolution, or emergencies.
+
+The main checkout may only be used for read-only inspection, fetching,
+stable-branch update after merged PRs, creating `branches/` worktrees, or
+explicit control-checkout repair.
+
+Portable wording: [`skills/llm-project-workflow/SKILL.md`](../skills/llm-project-workflow/SKILL.md).
+
 ## Branch worktree isolation
 
 All LLM implementation and review work happens in an isolated branch worktree