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/cmd/sling_helpers.go

@@ -10,6 +10,7 @@ import (
 
 	"github.com/steveyegge/gastown/internal/beads"
 	"github.com/steveyegge/gastown/internal/constants"
+	"github.com/steveyegge/gastown/internal/git"
 	"github.com/steveyegge/gastown/internal/tmux"
 	"github.com/steveyegge/gastown/internal/workspace"
 )
@@ -331,13 +332,13 @@ func ensureAgentReady(sessionName string) error {
 }
 
 // detectCloneRoot finds the root of the current git clone.
+// Uses cached value to avoid repeated git subprocess calls.
 func detectCloneRoot() (string, error) {
-	cmd := exec.Command("git", "rev-parse", "--show-toplevel")
-	out, err := cmd.Output()
+	root, err := git.RepoRoot()
 	if err != nil {
 		return "", fmt.Errorf("not in a git repository")
 	}
-	return strings.TrimSpace(string(out)), nil
+	return root, nil
 }
 
 // detectActor returns the current agent's actor string for event logging.