.claude: add backport-pr-assistant skill for conflict resolution

Add a Claude Code skill that assists with backporting PRs to release
branches when merge conflicts require manual resolution. The skill
provides reference documentation for the backport CLI tool and
guidelines for resolving conflicts during the backport process.

This is implemented as a skill (rather than an agent) because simple
backports are handled by automation, so Claude is only invoked for
cases with conflicts that require interactive discussion and
resolution.

The skill covers:
- Backport CLI tool usage and common commands
- Workflow for handling conflicts during backports
- Guidelines for simple vs complex conflict resolution
- Quality checks after successful backport

Epic: None
Release note: None

Author: rafiss

.claude/skills/backport-pr-assistant/SKILL.md

@@ -0,0 +1,66 @@
+---
+name: backport-pr-assistant
+description: Help backport PRs to release branches using the backport CLI tool. Use when backporting changes that have merge conflicts requiring manual resolution.
+---
+
+# CockroachDB Backport Assistant
+
+Help the user backport pull requests to older release branches, especially when conflicts need resolution.
+
+## Backport CLI Tool Reference
+
+**Basic Usage:**
+```bash
+backport <pull-request>...              # Backport entire PR(s)
+backport <pr> -r <release>              # Target specific release (e.g., -r 23.2)
+backport <pr> -b <branch>               # Target specific branch (e.g., -b release-23.1.10-rc)
+backport <pr> -j "justification"        # Add release justification
+backport <pr> -c <commit> -c <commit>   # Cherry-pick specific commits only
+backport <pr> -f                        # Force operation
+```
+
+**Conflict Resolution:**
+```bash
+backport --continue                     # Resume after resolving conflicts
+backport --abort                        # Cancel in-progress backport
+```
+
+**Common Examples:**
+```bash
+backport 23437                          # Simple backport
+backport 23437 -r 23.2                  # To release-23.2 branch
+backport 23437 -j "test-only changes"   # With justification
+backport 23437 -b release-23.1.10-rc    # To specific release candidate branch
+```
+
+## Workflow
+
+1. **Start the backport**: Run `backport <pr> -r <release>` for the target branch
+2. **When conflicts occur**: The tool stops and lists conflicting files
+3. **Analyze conflicts**: Read the conflicting files, understand what's different between branches
+4. **Resolve conflicts**: Edit files to resolve, then `git add` the resolved files
+5. **Continue**: Run `backport --continue` to resume
+6. **Repeat** if more conflicts arise
+7. **Complete**: The backport tool pushes and creates the PR (do not use `gh` CLI to make the PR)
+
+## Conflict Resolution Guidelines
+
+**Simple conflicts you can resolve directly:**
+- Import statement conflicts
+- Simple variable name changes
+- Basic formatting differences
+- Minor API signature changes that are obvious
+
+**Complex conflicts - ask the user for guidance:**
+- Conflicts involving significant logic changes
+- Dependencies that don't exist in the target branch
+- API changes requiring substantial modification
+- Multiple conflicting files with interdependent changes
+- Changes that may not be appropriate for the target branch
+
+## When Resolving Conflicts
+
+1. **Explain what's conflicting** - show the relevant code sections
+2. **Explain why** - what's different between branches that caused this
+3. **Propose a resolution** - or ask for guidance if complex
+4. **After resolving**: `git add <files>` then `backport --continue`