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/version/stale.go

@@ -7,6 +7,8 @@ import (
 	"os/exec"
 	"runtime/debug"
 	"strings"
+
+	"github.com/steveyegge/gastown/internal/git"
 )
 
 // These variables are set at build time via ldflags in cmd package.
@@ -133,9 +135,8 @@ func GetRepoRoot() (string, error) {
 	}
 
 	// Check if current directory is in a gastown repo
-	cmd := exec.Command("git", "rev-parse", "--show-toplevel")
-	if output, err := cmd.Output(); err == nil {
-		root := strings.TrimSpace(string(output))
+	// Uses cached git.RepoRoot() to avoid repeated subprocess calls
+	if root, err := git.RepoRoot(); err == nil {
 		if hasGastownMarker(root) {
 			return root, nil
 		}