feat(mq): implement integration land command (gt-h5n.5)

Add 'gt mq integration land <epic>' command that:
- Verifies all MRs targeting integration/<epic> are merged
- Verifies integration branch exists
- Merges integration/<epic> to main (--no-ff)
- Runs tests on main (if configured)
- Pushes to origin
- Deletes integration branch (local and remote)
- Updates epic status to closed

Options:
- --force: land even if some MRs still open
- --skip-tests: skip test run
- --dry-run: preview only

Also adds:
- MergeNoFF() and DeleteRemoteBranch() to git package
- WorkDir() accessor for git.Git
- Unit tests for mq helper functions

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

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

internal/git/git.go

@@ -27,6 +27,11 @@ func NewGit(workDir string) *Git {
 	return &Git{workDir: workDir}
 }
 
+// WorkDir returns the working directory for this Git instance.
+func (g *Git) WorkDir() string {
+	return g.workDir
+}
+
 // run executes a git command and returns stdout.
 func (g *Git) run(args ...string) (string, error) {
 	cmd := exec.Command("git", args...)
@@ -201,6 +206,18 @@ func (g *Git) Merge(branch string) error {
 	return err
 }
 
+// MergeNoFF merges the given branch with --no-ff flag and a custom message.
+func (g *Git) MergeNoFF(branch, message string) error {
+	_, err := g.run("merge", "--no-ff", "-m", message, branch)
+	return err
+}
+
+// DeleteRemoteBranch deletes a branch on the remote.
+func (g *Git) DeleteRemoteBranch(remote, branch string) error {
+	_, err := g.run("push", remote, "--delete", branch)
+	return err
+}
+
 // Rebase rebases the current branch onto the given ref.
 func (g *Git) Rebase(onto string) error {
 	_, err := g.run("rebase", onto)