chore(codeql): document empty fallback catches

[saagpatel]

What

• Adds explanatory comments to intentional empty fallback catches across analyzers, web routes, CLI fallbacks, exports, and targeted tests.
• Documents one additional non-alert source fallback to avoid leaving the same pattern behind.

Why

• Clears the largest remaining CodeQL note-level cluster: py/empty-except.
• Makes best-effort fallback behavior easier to audit without changing runtime behavior.

How

• Keeps existing behavior intact and explains each intentional swallow point in place.

Testing

• python3 -m pytest tests/test_serve.py tests/test_plan_campaign.py tests/test_draft_readmes.py tests/test_analyzers.py tests/test_readme_suggestions.py tests/test_issue_creator.py tests/test_excel_export_registry_helpers.py tests/test_cloner.py tests/test_briefing.py tests/test_web_export.py -q -p no:cacheprovider — 276 passed, 1 warning
• ruff check src/ tests/test_plan_campaign.py tests/test_draft_readmes.py — passed
• source scan for uncommented except: pass patterns — no matches in src/
• python3 -m pytest -q -p no:cacheprovider — 2085 passed, 2 skipped, 1 warning
• ruff check src/ tests/ — passed

Performance Impact

• None expected; comments only.

Risk / Notes

• This is intentionally documentation-only for existing fallback behavior.