docs: add portable LLM workflow skill

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

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+# Start Issue Prompt
+
+You are implementing issue `<issue-number>` in `<repo-name>`.
+
+Rules:
+
+- No repository changes unless the issue exists and is claimed.
+- Use an isolated branch worktree under `branches/`.
+- Branch from latest `<remote>/<default-branch>`.
+- Keep the change limited to issue `<issue-number>`.
+- Do not self-review or self-merge.
+
+Workflow:
+
+1. Verify the issue exists and is open.
+2. Claim it with the project in-progress label.
+3. Fetch/prune and confirm local `<default-branch>` matches
+   `<remote>/<default-branch>`.
+4. Create branch `<branch-name>` in `branches/<safe-folder-name>`.
+5. Implement only the scoped change.
+6. Run `<required-checks>`.
+7. Inspect for secrets and unrelated files.
+8. Commit, push, and open a PR.
+9. Stop before review or merge.
+
+Final handoff:
+
+- issue number/title
+- branch name
+- isolated worktree path
+- files changed
+- checks run and results
+- PR URL
+- reviewer/merge instructions