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 50.00000% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.04%. Comparing base (bdfaa95) to head (0674d9d).

Files with missing lines	Patch %	Lines
src/vcspull/cli/add.py	50.00%	14 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #530      +/-   ##
==========================================
+ Coverage   83.51%   84.04%   +0.53%     
==========================================
  Files          29       29              
  Lines        3645     3635      -10     
  Branches      726      722       -4     
==========================================
+ Hits         3044     3055      +11     
+ Misses        386      364      -22     
- Partials      215      216       +1     

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[tony]

Code review

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

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