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/prime.go

@@ -11,6 +11,7 @@ import (
 
 	"github.com/spf13/cobra"
 	"github.com/steveyegge/gastown/internal/beads"
+	"github.com/steveyegge/gastown/internal/git"
 	"github.com/steveyegge/gastown/internal/lock"
 	"github.com/steveyegge/gastown/internal/state"
 	"github.com/steveyegge/gastown/internal/style"
@@ -527,13 +528,9 @@ func buildRoleAnnouncement(ctx RoleContext) string {
 }
 
 // getGitRoot returns the root of the current git repository.
+// Uses cached value to avoid repeated git subprocess calls.
 func getGitRoot() (string, error) {
-	cmd := exec.Command("git", "rev-parse", "--show-toplevel")
-	out, err := cmd.Output()
-	if err != nil {
-		return "", err
-	}
-	return strings.TrimSpace(string(out)), nil
+	return git.RepoRoot()
 }
 
 // getAgentIdentity returns the agent identity string for hook lookup.