dx(skills): add /open-pr and /setup-repo skills (#12591)

### Why / What / How

**Why:** Agents working in worktrees lack guidance on two of the most
common workflows: properly opening PRs (using the repo template,
validating test coverage, triggering the review bot) and bootstrapping
the repo from scratch with a worktree-based layout. Without these
skills, agents either skip steps (no test plan, wrong template) or
require manual hand-holding for setup.

**What:** Adds two new Claude Code skills under `.claude/skills/`:
- `/open-pr` — A structured PR creation workflow that enforces the
canonical `.github/PULL_REQUEST_TEMPLATE.md`, validates test coverage
for existing and new behaviors, supports a configurable base branch, and
integrates the `/review` bot workflow for agents without local testing
capability. Cross-references `/pr-test`, `/pr-review`, and `/pr-address`
for the full PR lifecycle.
- `/setup-repo` — An interactive repo bootstrapping skill that creates a
worktree-based layout (main + reviews + N numbered work branches).
Handles .env file provisioning with graceful fallbacks (.env.default,
.env.example), copies branchlet config, installs dependencies, and is
fully idempotent (safe to re-run).

**How:** Markdown-based SKILL.md files following the existing skill
conventions. Both skills use proper bash patterns (seq-based loops
instead of brace expansion with variables, existence checks before
branch/worktree creation, error reporting on install failures).
`/open-pr` delegates to AskUserQuestion-style prompts for base branch
selection. `/setup-repo` uses AskUserQuestion for interactive branch
count and base branch selection.

### Changes 🏗️

- Added `.claude/skills/open-pr/SKILL.md` — PR creation workflow with:
  - Pre-flight checks (committed, pushed, formatted)
- Test coverage validation (existing behavior not broken, new behavior
covered)
- Canonical PR template enforcement (read and fill verbatim, no
pre-checked boxes)
  - Configurable base branch (defaults to dev)
- Review bot workflow (`/review` comment + 30min wait) for agents
without local testing
  - Related skills table linking `/pr-test`, `/pr-review`, `/pr-address`

- Added `.claude/skills/setup-repo/SKILL.md` — Repo bootstrap workflow
with:
- Interactive setup (branch count: 4/8/16/custom, base branch selection)
- Idempotent branch creation (skips existing branches with info message)
  - Idempotent worktree creation (skips existing directories)
- .env provisioning with fallback chain (.env → .env.default →
.env.example → warning)
  - Branchlet config propagation
  - Dependency installation with success/failure reporting per worktree

### Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
  - [x] Verified SKILL.md frontmatter follows existing skill conventions
  - [x] Verified trigger conditions match expected user intents
  - [x] Verified cross-references to existing skills are accurate
- [x] Verified PR template section matches
`.github/PULL_REQUEST_TEMPLATE.md`
- [x] Verified bash snippets use correct patterns (seq, show-ref, quoted
vars)
  - [x] Pre-commit hooks pass on all commits
  - [x] Addressed all CodeRabbit, Sentry, and Cursor review comments

🤖 Generated with [Claude Code](https://claude.com/claude-code)

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Low risk documentation-only change: adds new markdown skills without
modifying runtime code. Main risk is workflow guidance drift (e.g.,
`.env`/worktree steps) if it diverges from actual repo conventions.
> 
> **Overview**
> Adds two new Claude Code skills under `.claude/skills/` to standardize
common developer workflows.
> 
> `/open-pr` documents a PR creation flow that enforces using
`.github/PULL_REQUEST_TEMPLATE.md` verbatim, calls out required test
coverage, and describes how to trigger/poll the `/review` bot when local
testing isn’t available.
> 
> `/setup-repo` documents an idempotent, interactive bootstrap for a
multi-worktree layout (creates `reviews` and `branch1..N`, provisions
`.env` files with `.env.default`/`.env.example` fallbacks, copies
`.branchlet.json`, and installs dependencies), complementing the
existing `/worktree` skill.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
80dbeb15964f1f536134dc82b57ddd37a5a3273c. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: Claude Opus 4.6 (1M context) <[email protected]>
Co-authored-by: copilot-swe-agent[bot] <[email protected]>