vcspull fmt: Detect and merge identical roots, snapshot tests (#479)

Summary:
- enable `vcspull fmt` to detect duplicate workspace roots and merge entries by
  default, logging detailed normalization hints
- introduce `--no-merge` to retain duplicate sections while still reporting
  diagnostics for manual review
- run formatting across all discovered configs (`--all`) with merge control,
  reusing the new logic
- switch formatter unit and CLI tests to Syrupy snapshots (`snapshot_json` and
  custom `snapshot_yaml`) so dictionary ordering and YAML output stay stable
- document the change in CHANGES under Improvements and Development, linking to
  PR #479

Testing:
- uv run ruff check . --fix --show-fixes
- uv run ruff format .
- uv run mypy
- uv run py.test

Author: tony

CHANGES

@@ -33,6 +33,24 @@ $ uvx --from 'vcspull' --prerelease allow vcspull
 
 _Upcoming changes will be written here._
 
+### Improvements
+
+#### `vcspull fmt` gains duplicate root merging (#479)
+
+- Detects repeated workspace root labels and merges their repositories during
+  formatting so users keep a single normalized section.
+- Adds `--no-merge` for workflows that need to preserve duplicate entries while
+  still seeing diagnostic warnings.
+
+### Development
+
+#### Snapshot coverage for formatter tests (#479)
+
+- Formatter scenarios now use [Syrupy]-backed JSON and YAML snapshots to guard
+  against regressions in duplicate workspace-root merging.
+
+[syrupy]: https://github.com/syrupy-project/syrupy
+
 ## vcspull v1.42.0 (2025-11-01)
 
 ### Breaking changes

pyproject.toml

@@ -85,6 +85,7 @@ dev = [
   # Testing
   "gp-libs",
   "pytest",
+  "syrupy",
   "pytest-asyncio",
   "pytest-rerunfailures",
   "pytest-mock",
@@ -119,6 +120,7 @@ docs = [
 testing = [
   "gp-libs",
   "pytest",
+  "syrupy",
   "pytest-asyncio",
   "pytest-rerunfailures",
   "pytest-mock",

src/vcspull/cli/__init__.py

@@ -386,4 +386,9 @@ def cli(_args: list[str] | None = None) -> None:
             dry_run=args.dry_run,
         )
     elif args.subparser_name == "fmt":
-        format_config_file(args.config, args.write, args.all)
+        format_config_file(
+            args.config,
+            args.write,
+            args.all,
+            merge_roots=args.merge_roots,
+        )