feat(mq): add gt mq integration create command

Implement 'gt mq integration create <epic>' command to create integration
branches for batch work on epics. The command:

1. Verifies the epic exists in beads
2. Creates branch integration/<epic-id> from origin/main
3. Pushes the branch to origin
4. Stores integration branch info in the epic's metadata

Also adds helper methods to git package:
- CreateBranchFrom: create branch from specific ref
- BranchExists: check if local branch exists
- RemoteBranchExists: check if branch exists on remote

Future MRs for the epic's children can target the integration branch
with: gt mq submit --epic <epic-id>

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/git/git.go

@@ -314,6 +314,39 @@ func (g *Git) CreateBranch(name string) error {
 	return err
 }
 
+// CreateBranchFrom creates a new branch from a specific ref.
+func (g *Git) CreateBranchFrom(name, ref string) error {
+	_, err := g.run("branch", name, ref)
+	return err
+}
+
+// BranchExists checks if a branch exists locally.
+func (g *Git) BranchExists(name string) (bool, error) {
+	_, err := g.run("show-ref", "--verify", "--quiet", "refs/heads/"+name)
+	if err != nil {
+		// Exit code 1 means branch doesn't exist
+		if strings.Contains(err.Error(), "exit status 1") {
+			return false, nil
+		}
+		return false, err
+	}
+	return true, nil
+}
+
+// RemoteBranchExists checks if a branch exists on the remote.
+func (g *Git) RemoteBranchExists(remote, branch string) (bool, error) {
+	_, err := g.run("ls-remote", "--heads", remote, branch)
+	if err != nil {
+		return false, err
+	}
+	// ls-remote returns empty if branch doesn't exist, need to check output
+	out, err := g.run("ls-remote", "--heads", remote, branch)
+	if err != nil {
+		return false, err
+	}
+	return out != "", nil
+}
+
 // DeleteBranch deletes a branch.
 func (g *Git) DeleteBranch(name string, force bool) error {
 	flag := "-d"