refactor(cli[add]) Extract duplicated save-config dispatch helpers

[tony]

Summary

Closes #521

• Extract _save_config_or_log_error() for the try/except/log.exception pattern (used 4x)
• Extract _extract_current_url() for deriving display URLs from config entries (duplicated in merge/no-merge paths)
• Extract _handle_skip_action() for SKIP_PINNED/SKIP_EXISTING branches (~30 lines each, duplicated)
• Both merge and no-merge paths now use local closures + shared helpers

Test plan

• uv run ruff check . — all checks passed
• uv run ruff format . — no changes needed
• uv run mypy — no issues found
• uv run py.test -vvv — 1175 passed

[codecov]

Codecov Report

❌ Patch coverage is 68.00000% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.79%. Comparing base (e6a62d9) to head (02fd15e).

Files with missing lines	Patch %	Lines
src/vcspull/cli/add.py	68.00%	2 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #530      +/-   ##
==========================================
+ Coverage   83.30%   83.79%   +0.48%     
==========================================
  Files          32       32              
  Lines        4492     4467      -25     
  Branches      903      902       -1     
==========================================
+ Hits         3742     3743       +1     
+ Misses        499      471      -28     
- Partials      251      253       +2     

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[tony]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

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[tony]

Re-authored on current master. The original branch collided with master's own add.py refactor (the AddAction SKIP_PINNED/SKIP_EXISTING split), which had increased the duplication this PR targets — there are now six identical save-with-error-handling blocks and two identical current-URL derivations. Kept the PR's helper primitives (_save_config_or_log_error, _extract_current_url) and routed all six save sites + both URL derivations through them, preserving the merge vs no-merge dispatcher (save_config vs _save_ordered_items) via a passed closure. Both helpers carry doctests plus parametrized functional tests. Full gate (ruff/mypy/pytest/build-docs) green.