Make output_modules additive, add exclude_output_modules

[liquidsec]

Summary

• output_modules is now purely additive -- -om discord adds discord on top of defaults (csv, txt, json), never replaces them. The old override hack in bake() silently dropped defaults when any default module name appeared in the list, which was confusing and inconsistent.
• New -eom / --exclude-output-modules flag (and exclude_output_modules preset key / Python API param) for explicitly removing output modules, including defaults.
• Moved python module from output/ to internal/ -- it was never a real output module; it's the Python API event bridge. Still inherits BaseOutputModule for behavioral properties (accept_dupes, scope_distance_modifier=None).
• Fixed bake() safety check that silently re-added excluded default output modules.
• Updated docs (output.md, index.md, dev/index.md) to document additive behavior and -eom.

Closes #3146

[github-actions]

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[codecov]

Codecov Report

❌ Patch coverage is 96.29630% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90%. Comparing base (ef65b09) to head (b2f53f8).

Files with missing lines	Patch %	Lines
bbot/scanner/preset/preset.py	89%	2 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##             dev   #3221   +/-   ##
=====================================
+ Coverage     90%     90%   +1%     
=====================================
  Files        453     453           
  Lines      46101   46127   +26     
=====================================
+ Hits       41211   41237   +26     
  Misses      4890    4890           

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