refactor(cli) Remove redundant traceback + extract MockImporter

[tony]

Summary

Commit 1: Remove redundant traceback.print_exc() after log.exception()

• log.exception() already includes the full traceback in logging output
• Removed 11 redundant traceback.print_exc() blocks across add.py (5), discover.py (3), fmt.py (2)
• sync.py had a different case (traceback.print_exc() after log.debug()): replaced with idiomatic log.debug(..., exc_info=True)
• Removed unused import traceback from 3 files

Commit 2: Extract shared MockImporter to tests/cli/conftest.py

• Moved MockImporter and CapturingMockImporter from test_import_repos.py to tests/cli/conftest.py
• Updated test_import_repos.py to import from conftest

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 — 1172 passed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.03%. Comparing base (bdfaa95) to head (d78e9a2).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #531      +/-   ##
==========================================
+ Coverage   83.51%   84.03%   +0.52%     
==========================================
  Files          29       29              
  Lines        3645     3620      -25     
  Branches      726      715      -11     
==========================================
- Hits         3044     3042       -2     
+ Misses        386      366      -20     
+ Partials      215      212       -3     

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[tony]

Code review

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

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