vcspull fmt: Detect and merge identical roots

[tony]

Changes

Formatter Enhancements

• detect duplicate workspace roots during vcspull fmt and merge them by default
• add --no-merge flag to keep duplicate sections when needed; e.g. vcspull fmt --no-merge

Test Coverage

• adopt Syrupy JSON snapshots for format_config outputs via snapshot_json
• persist YAML results with a custom Syrupy extension (snapshot_yaml) to freeze on-disk formatting; e.g. pytest tests/cli/test_fmt.py --snapshot-update

Test Plan

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

[codecov]

Codecov Report

❌ Patch coverage is 79.04762% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.70%. Comparing base (cab8fd9) to head (0456d73).
⚠️ Report is 9 commits behind head on master.

Files with missing lines	Patch %	Lines
src/vcspull/cli/fmt.py	79.04%	12 Missing and 10 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #479   +/-   ##
=======================================
  Coverage   78.70%   78.70%           
=======================================
  Files          13       13           
  Lines        1498     1592   +94     
  Branches      321      341   +20     
=======================================
+ Hits         1179     1253   +74     
- Misses        201      213   +12     
- Partials      118      126    +8     

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