feat(hooks): add jujutsu (jj) version control support

Add detection and hook support for jujutsu repositories:

- IsJujutsuRepo(): detects .jj directory
- IsColocatedJJGit(): detects colocated jj+git repos
- GetJujutsuRoot(): finds jj repo root

For colocated repos (jj git init --colocate):
- Install simplified hooks without staging (jj auto-commits working copy)
- Worktree handling preserved for git worktrees in colocated repos

For pure jj repos (no git):
- Print alias instructions since jj doesn't have native hooks yet

Closes: hq-ew1mbr.12

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

internal/git/gitdir.go

@@ -2,6 +2,7 @@ package git
 
 import (
 	"fmt"
+	"os"
 	"os/exec"
 	"path/filepath"
 	"runtime"
@@ -271,3 +272,45 @@ func ResetCaches() {
 	gitCtxOnce = sync.Once{}
 	gitCtx = gitContext{}
 }
+
+// IsJujutsuRepo returns true if the current directory is in a jujutsu (jj) repository.
+// Jujutsu stores its data in a .jj directory at the repository root.
+func IsJujutsuRepo() bool {
+	_, err := GetJujutsuRoot()
+	return err == nil
+}
+
+// IsColocatedJJGit returns true if this is a colocated jujutsu+git repository.
+// Colocated repos have both .jj and .git directories, created via `jj git init --colocate`.
+// In colocated repos, git hooks work normally since jj manages the git repo.
+func IsColocatedJJGit() bool {
+	if !IsJujutsuRepo() {
+		return false
+	}
+	// If we're also in a git repo, it's colocated
+	_, err := getGitContext()
+	return err == nil
+}
+
+// GetJujutsuRoot returns the root directory of the jujutsu repository.
+// Returns empty string and error if not in a jujutsu repository.
+func GetJujutsuRoot() (string, error) {
+	cwd, err := os.Getwd()
+	if err != nil {
+		return "", fmt.Errorf("failed to get current directory: %w", err)
+	}
+
+	dir := cwd
+	for {
+		jjPath := filepath.Join(dir, ".jj")
+		if info, err := os.Stat(jjPath); err == nil && info.IsDir() {
+			return dir, nil
+		}
+
+		parent := filepath.Dir(dir)
+		if parent == dir {
+			return "", fmt.Errorf("not a jujutsu repository (no .jj directory found)")
+		}
+		dir = parent
+	}
+}