perf(git): cache git rev-parse results within sessions

Multiple gt commands call git rev-parse --show-toplevel, adding ~50ms
each invocation. Results rarely change within a session, and multiple
agents calling git concurrently contend on .git/index.lock.

Add cached RepoRoot() and RepoRootFrom() functions to the git package
and update all callers to use them. This ensures a single git subprocess
call per process for the common case of checking the current directory's
repo root.

Files updated:
- internal/git/git.go: Add RepoRoot() and RepoRootFrom()
- internal/cmd/prime.go: Use cached git.RepoRoot()
- internal/cmd/molecule_status.go: Use cached git.RepoRoot()
- internal/cmd/sling_helpers.go: Use cached git.RepoRoot()
- internal/cmd/rig_quick_add.go: Use git.RepoRootFrom() for path arg
- internal/version/stale.go: Use cached git.RepoRoot()

Closes: bd-2zd.5

internal/git/git.go

@@ -9,8 +9,49 @@ import (
 	"os/exec"
 	"path/filepath"
 	"strings"
+	"sync"
 )
 
+// Cached repo root for the current process.
+// Since CLI commands are short-lived and the working directory doesn't change
+// during a single invocation, caching this avoids repeated git subprocess calls
+// that add ~50ms each and contend on .git/index.lock.
+var (
+	cachedRepoRoot     string
+	cachedRepoRootOnce sync.Once
+	cachedRepoRootErr  error
+)
+
+// RepoRoot returns the root directory of the git repository containing the
+// current working directory. The result is cached for the lifetime of the process.
+// This avoids repeated git rev-parse calls that are expensive (~50ms each) and
+// can cause lock contention when multiple agents are running.
+func RepoRoot() (string, error) {
+	cachedRepoRootOnce.Do(func() {
+		cmd := exec.Command("git", "rev-parse", "--show-toplevel")
+		out, err := cmd.Output()
+		if err != nil {
+			cachedRepoRootErr = err
+			return
+		}
+		cachedRepoRoot = strings.TrimSpace(string(out))
+	})
+	return cachedRepoRoot, cachedRepoRootErr
+}
+
+// RepoRootFrom returns the root directory of the git repository containing the
+// specified path. Unlike RepoRoot(), this is not cached because it depends on
+// the input path. Use RepoRoot() when checking the current working directory.
+func RepoRootFrom(path string) (string, error) {
+	cmd := exec.Command("git", "rev-parse", "--show-toplevel")
+	cmd.Dir = path
+	out, err := cmd.Output()
+	if err != nil {
+		return "", err
+	}
+	return strings.TrimSpace(string(out)), nil
+}
+
 // GitError contains raw output from a git command for agent observation.
 // ZFC: Callers observe the raw output and decide what to do.
 // The error interface methods provide human-readable messages, but agents