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/git-manager/git_cmd.bats

@@ -109,6 +109,23 @@ setup() {
   [[ "$output" == *"== Repo: $REPO =="* ]]
 }
 
+@test "a registered path that is a file, not a directory, errors" {
+  FILE_PATH="$BATS_TEST_TMPDIR/not-a-dir"
+  echo "just a file" > "$FILE_PATH"
+  write_git_manager_config "$CONFIG_FILE" "plain=$FILE_PATH"
+  run "$SCRIPT" status --repo plain
+  [ "$status" -eq 1 ]
+  [[ "$output" == *"is not a directory"* ]]
+}
+
+@test "missing python3 fails with a clear error instead of a silent crash" {
+  write_git_manager_config "$CONFIG_FILE" "myrepo=$REPO"
+  hide_command python3
+  run "$SCRIPT" status --repo myrepo
+  [ "$status" -eq 1 ]
+  [[ "$output" == *"python3 is required"* ]]
+}
+
 # --- -- passthrough ---
 
 @test "args after -- reach git unmodified" {