feat: add restack skill to guide through git restack conflicts

.claude/skills/restack/SKILL.md

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+# Restack
+
+Use when user asks to "restack", "resolve conflicts", "gt restack", or mentions restack conflicts. Guides through gt restack conflicts with intelligent diagnostics and resolution. (project)
+
+<critical>
+- ALWAYS check current state first (supports both: starting fresh or mid-conflict)
+- If already in conflict, skip running gt restack and go straight to resolution
+- Differentiate simple (lockfile-only) vs complex (multi-file) conflicts
+- Loop until restack completes successfully
+- Use AskUserQuestion for confirmations and choices
+</critical>
+
+## Workflow
+
+### 1. Detect Current State
+
+First, check if restack is already in progress:
+
+```bash
+git status
+```
+
+**Check for**:
+- "rebase in progress"
+- "You are currently rebasing"
+- "Unmerged paths"
+- "both modified:" or other conflict indicators
+
+**Outcomes**:
+- Already in conflict (user ran `gt restack` manually) → Skip to step 3 (Gather Diagnostics)
+- Clean state → Proceed to step 2 (Run Restack)
+
+### 2. Run Restack (if not already in progress)
+
+```bash
+gt restack
+```
+
+**Outcomes**:
+- Success → Acknowledge and exit
+- Conflicts → Proceed to step 3
+
+### 3. Gather Diagnostics
+
+When conflicts are detected (either from step 1 or step 2), run:
+
+```bash
+# Check which files are conflicted
+git status
+
+# Show stack structure (where conflict occurred)
+gt log short
+
+# Check for broken lockfile warning
+pnpm install --dry-run 2>&1 | grep -i "broken\|duplicated\|ignoring"
+
+# For each conflicted file, show diff
+git diff <file>
+```
+
+### 3. Explain Conflict Type
+
+**Simple conflict** (lockfile-only):
+- Only `pnpm-lock.yaml` in unmerged files
+- Explain: Lockfile is generated content, safe to regenerate
+- Propose: `pnpm install && gt add -A && gt continue`
+
+**Complex conflict** (multi-file):
+- Multiple files in unmerged files
+- Explain each file's conflict
+- Guide through resolution strategy
+
+### 4. Resolve Based on Type
+
+#### Simple Conflict Resolution
+
+Use AskUserQuestion:
+```
+question: "Only pnpm-lock.yaml is conflicted. Regenerate and continue?"
+header: "Simple Resolution"
+options:
+  - label: "Yes, run one-liner"
+    description: "pnpm install && gt add -A && gt continue"
+  - label: "Let me inspect first"
+    description: "Show git diff pnpm-lock.yaml before proceeding"
+```
+
+If confirmed → Execute → Check output of `gt continue`:
+- Another conflict → Loop back to step 3 (Gather Diagnostics)
+- Success message → Proceed to step 6 (Success & Reminder)
+
+#### Complex Conflict Resolution
+
+For each non-lockfile conflict:
+
+**a) Show the conflict**:
+```bash
+git diff <file>
+```
+
+**b) Explain the conflict** (extract from diff):
+- What changed in base (HEAD)
+- What changed in branch (incoming)
+- Why it conflicts
+
+**c) Ask resolution strategy** using AskUserQuestion:
+```
+question: "How to resolve <file>?"
+header: "Resolution"
+options:
+  - label: "--theirs (main's version)"
+    description: "Accept base branch - good for style/formatting consistency"
+  - label: "--ours (branch's version)"
+    description: "Keep your branch changes - good for features/dependencies"
+  - label: "Manual edit"
+    description: "I'll resolve it manually, just continue after I'm done"
+  - label: "Show original file"
+    description: "Show git show HEAD:<file> and git show :<stage>:<file> to inspect"
+```
+
+**d) Execute choice**:
+- `--theirs`: `git checkout --theirs <file>`
+- `--ours`: `git checkout --ours <file>`
+- Manual: Wait for user confirmation
+- Show original: Display and ask again
+
+**e) After all non-lockfile conflicts resolved**:
+```bash
+pnpm install && gt add -A && gt continue
+```
+
+**f) Check output of `gt continue`**:
+- Another conflict → Loop back to step 3 (Gather Diagnostics)
+- Success message → Proceed to step 6 (Success & Reminder)
+
+### 5. Handle Broken Lockfile
+
+If diagnostics show "broken lockfile" warning:
+
+Use AskUserQuestion:
+```
+question: "Lockfile is corrupted. Delete and regenerate?"
+header: "Broken Lockfile"
+options:
+  - label: "Yes, delete and regenerate"
+    description: "rm pnpm-lock.yaml && pnpm install && gt add -A && gt continue"
+  - label: "Try pnpm install first"
+    description: "pnpm might auto-fix, try without deleting first"
+```
+
+After executing choice, check output of command:
+- Another conflict → Loop back to step 3 (Gather Diagnostics)
+- Success message → Proceed to step 6 (Success & Reminder)
+
+### 6. Success & Reminder
+
+When `gt restack` completes without conflicts:
+
+```
+Restack completed successfully!
+
+Next step: Force push your stack
+  gt stack submit --force
+  # Or individual branch:
+  git push --force-with-lease origin <branch-name>
+
+Note: Always use --force-with-lease for safety
+```
+
+## Quick Reference
+
+**Simple lockfile conflict**:
+```bash
+pnpm install && gt add -A && gt continue
+```
+
+**Multiple files conflicted**:
+```bash
+git diff <file>                    # Inspect
+git checkout --ours/--theirs <file>  # Resolve
+pnpm install && gt add -A && gt continue
+```
+
+**Broken lockfile**:
+```bash
+rm pnpm-lock.yaml
+pnpm install && gt add -A && gt continue
+```
+
+**Cancel restack**:
+```bash
+gt abort
+```
+
+## Resolution Strategy Guide
+
+**When to use `--theirs` (accept main's version)**:
+- Style/formatting conflicts (e.g., quoted vs unquoted YAML)
+- Configuration files where consistency with main matters
+- Files you didn't intentionally change
+
+**When to use `--ours` (keep branch's version)**:
+- Your branch added dependencies (e.g., pnpm-workspace.yaml)
+- Feature changes you need to preserve
+- Files where your changes are the whole point
+
+**Rule of thumb**: When in doubt, choose consistency with main (`--theirs`).
+
+## Troubleshooting
+
+**Problem**: `gt continue` fails with error
+**Solution**: Check `git status` for unresolved conflicts or unstaged files
+
+**Problem**: pnpm install shows warnings about bin files
+**Solution**: Usually safe to ignore - supabase CLI has known installation quirks
+
+**Problem**: Not sure whether to use --ours or --theirs
+**Solution**: Skill will show both versions and explain - use AskUserQuestion to decide
+
+**Problem**: Multiple files conflicted, unclear which to resolve first
+**Solution**: Skill resolves non-lockfiles first, then regenerates lockfile last