noteUntrackedSkipped: fix subdir underreport + be best-effort everywhere

Two bugs in the untracked-skipped warning, both surfaced by review.

1. Wrong scope for subdir inputs. The helper accepted any path the
   caller had (sourcePath, which may be a user-supplied subdirectory)
   and ran `git -C <path> ls-files --others --exclude-standard`. Git
   scopes that output to the cwd, so pointing `vm run ./repo/sub` at
   a subdir silently underreported untracked files living elsewhere
   in the repo — exactly the files the warning exists to surface.
   Fix: resolve sourcePath to the repo root inside the helper via
   `rev-parse --show-toplevel` before counting.

2. Inconsistent failure handling. The comment said the helper should
   be silent when the count can't be determined; the body returned
   the error. vm_run.go treated the error as non-fatal (logged a
   warning, continued); workspace prepare and --dry-run aborted the
   whole operation on the same helper failure. A courtesy notice
   shouldn't kill the operation.
   Fix: make the helper best-effort in signature and body — no error
   return, swallows rev-parse + count failures, emits nothing when
   there's nothing to say. All three callers lose their error
   branches.

Regression tests:
- subdir input reports the root-level untracked file (the bug case)
- non-repo path produces silence, not a fatal error
- inspector whose runner errors on every call produces silence

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/cli/commands_vm.go

@@ -625,9 +625,7 @@ func (d *deps) newVMWorkspacePrepareCommand() *cobra.Command {
 				return err
 			}
 			if !includeUntracked {
-				if err := d.noteUntrackedSkipped(cmd.Context(), cmd.ErrOrStderr(), resolvedPath); err != nil {
-					return err
-				}
+				d.noteUntrackedSkipped(cmd.Context(), cmd.ErrOrStderr(), resolvedPath)
 			}
 			result, err := d.vmWorkspacePrepare(cmd.Context(), layout.SocketPath, api.VMWorkspacePrepareParams{
 				IDOrName:         args[0],