Close the 3 real remaining coverage gaps

- vault_index.sh: grep fallback for frontmatter tag extraction when rg
  is unavailable (had this for vault_search.sh/vault_backlinks.sh, not
  vault_index.sh).
- git_cmd.bats: registered repo path that's a file, not a directory
  (had the obsidian equivalent, not git-manager's).
- git_cmd.bats: missing python3 in resolve_repo_path (previously only
  tested via setup.sh/vault_index.sh, not git_cmd.sh's own _lib.sh).

76 tests now (was 73). kcov: 87.84% (260/296 lines), up from 86.15% --
both target files (vault_index.sh, git-manager _lib.sh) now at 100%.
Remaining gap is entirely kcov's inability to trace multi-line
python3/awk/jq blocks and bash 'case pattern) ;;' no-op branches,
confirmed by checking hit counts on the surrounding lines -- not
further closeable by adding tests.

tests/obsidian-vault-kb/vault_index.bats

@@ -34,6 +34,15 @@ setup() {
   done
 }
 
+@test "grep fallback still extracts tags when rg is unavailable" {
+  hide_command rg
+  run "$SCRIPT" v
+  [ "$status" -eq 0 ]
+  for tag in dns homelab networking overview; do
+    [[ "$output" == *"$tag"* ]]
+  done
+}
+
 @test "a note matching *overview* is flagged as a possible MOC file" {
   run "$SCRIPT" v
   [ "$status" -eq 0 ]