Adopt WorkspaceRoot convention for root dirs, fix root ambiguity issues

[tony]

Summary

Motivation

• vcspull import --scan treated ~/study/c, ~/study/c/, and /home/user/study/c
  as three unrelated sections, so existing repositories were re-offered during
  scans.
• Formatter output could keep redundant workspace-root sections, allowing the
  duplication to persist after a format pass.
• CLI help, docs, and tests still mixed the legacy "base dir" terminology with
  the newer workspace-root language.

What changed

• Added canonicalization helpers that resolve workspace roots to an absolute
  pathlib.Path, collapse to a single label, and memoize the mapping for reuse
  across importer and formatter flows.
• Updated vcspull import to normalize both config files and newly discovered
  directories before merging, ensuring repeated scans do not present the same
  repositories twice.
• Extended vcspull fmt to call the same normalization routine prior to
  sorting/writing, so the formatter also removes duplicate sections and writes
  consistent labels.
• Replaced the --base-dir-key/--dir CLI options and internal parameters with
  --workspace-root, and refreshed CLI help, README, and docs to match.
• Expanded CLI test matrices (import+fmt) with NamedTuple-driven parametrization
  covering all combinations of tilde, absolute, trailing-slash, and relative
  workspace-root inputs.

Testing

• uv run ruff check . --fix --show-fixes
• uv run ruff format .
• uv run mypy
• uv run py.test
• uv run vcspull import --scan ~/study/c
• uv run vcspull fmt --write --all

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.26%. Comparing base (06af066) to head (7f1c938).
⚠️ Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
src/vcspull/config.py	78.00%	7 Missing and 4 partials ⚠️
src/vcspull/cli/fmt.py	60.00%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #470      +/-   ##
==========================================
- Coverage   80.23%   79.26%   -0.97%     
==========================================
  Files           9        9              
  Lines         597      656      +59     
  Branches      134      147      +13     
==========================================
+ Hits          479      520      +41     
- Misses         67       78      +11     
- Partials       51       58       +7     

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