chore(skills): add worktree-update skill

Non-destructive branch sync utility. On main: shows worktree branches
ahead of main with PR flags, lets user pick which to merge. On other
branches: merges main in. Stops on conflicts, never force-pushes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.claude/skills/worktree-update/SKILL.md

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+---
+name: worktree-update
+description: >
+  Sync worktree branches with main. Use when the user says "update worktrees",
+  "sync branches", "merge main", "worktree update", or invokes /worktree-update.
+  When on main: shows which worktree branches are ahead and lets the user pick
+  which to merge into main (flags branches with open PRs). When on a non-main
+  branch: merges main into the current branch. All operations are non-destructive.
+user-invocable: true
+allowed-tools: Bash, Read, AskUserQuestion
+---
+
+# Worktree Update Skill
+
+Sync worktree branches safely. Direction depends on the current branch.
+
+## Safety Rules (NON-NEGOTIABLE)
+
+- **Never force-push, reset --hard, rebase, or delete branches.**
+- **Never use `--no-verify` or skip hooks.**
+- **Always use `--no-edit` on merges** to avoid interactive editor prompts.
+- **Stop on merge conflicts** — report them and let the user decide. Never
+  auto-resolve or abort a conflicted merge without asking.
+- **Fetch before comparing** — always `git fetch --all` first so commit
+  comparisons are accurate.
+- **Dry-run first on main** — show the user exactly what will happen before
+  merging anything into main.
+
+## Workflow
+
+### 1. Detect current branch
+
+```bash
+git branch --show-current
+```
+
+Branch determines the mode: `main` → outbound sync, anything else → inbound sync.
+
+### 2a. On `main` — merge worktree branches into main
+
+#### Fetch and compare
+
+```bash
+git fetch --all
+```
+
+For each worktree branch, check if it has commits ahead of main:
+
+```bash
+git rev-list --count main..<branch>
+```
+
+Skip branches with 0 commits ahead. For branches that ARE ahead, collect:
+- Branch name
+- Number of commits ahead
+- One-line log of those commits: `git log --oneline main..<branch>`
+
+#### Check for open PRs
+
+```bash
+tea pr list --login schweitz --repo jpmschweitzer/settled-reach --state open --output simple
+```
+
+Cross-reference open PR head branches with the ahead-of-main branches.
+
+#### Present results
+
+Show a summary table of branches ahead of main. For each branch, indicate:
+- `[PR]` if it has an open pull request — warn that it should go through
+  normal review channels (use `/review-pr` instead)
+- Commit count and summary
+
+Use `AskUserQuestion` to let the user pick which branches to merge.
+Exclude PR-flagged branches from the default options (but allow the user to
+override via "Other").
+
+#### Merge selected branches
+
+For each selected branch, one at a time:
+
+```bash
+git merge <branch> --no-edit
+```
+
+If a merge conflicts, **stop immediately**. Report the conflict and do NOT
+continue to the next branch. The user must resolve before proceeding.
+
+After all merges, show the final state with `git log --oneline -N` (where N
+covers the new commits).
+
+### 2b. Not on `main` — merge main into current branch
+
+```bash
+git fetch --all
+git merge origin/main --no-edit
+```
+
+If clean, report the result (fast-forward or merge commit, files changed).
+If conflicts, report them and stop.
+
+### 3. Push prompt
+
+After a successful merge, ask the user if they want to push:
+
+```bash
+git push origin <current-branch>
+```
+
+Never push without explicit confirmation.