fix: address non-blocking reviewer nits from #140, #141, #143

[mimran-khan]

Summary

Bundles non-blocking reviewer nits from three merged PRs into a single follow-up:

• fix(static-patterns): filter false positives from documentation and code examples #140 (_NON_EXECUTABLE_FILE_TYPES): Remove "other" from the non-executable set so extensionless scripts (e.g. cleanup_script with no .sh extension, inferred as "other" by _infer_file_type) get code-example findings downweighted instead of hard-dropped. This closes the edge case @rng1995 flagged where an unknown-but-executable extension could re-open the code-example bypass.
• fix(meta-analyzer): add heuristic fallback filter for --no-llm mode #143 (_fallback_filtered severity guard): Guard f.severity.upper() with (f.severity or "LOW").upper() to match the codebase convention (e.g. _compute_risk_score). Prevents AttributeError if a finding has severity=None.
• feat(cli): add --recursive flag for multi-skill directory scanning #141 (--recursive help text): Reword to say "immediate subdirectories" instead of "directories containing multiple skills" — clarifies that --recursive scans one level deep, not a full recursive tree walk.

Testing

• test_extensionless_file_not_hard_dropped_by_code_example — confirms an extensionless file with code-example context is downweighted, not dropped.
• test_none_severity_treated_as_low — confirms severity=None doesn't crash and is treated as LOW.
• test_none_severity_below_threshold_dropped — confirms None severity at low confidence is dropped (no severity floor protection).
• All existing tests pass (43 in the affected test files).

Not addressed in this PR

• fix(meta-analyzer): add heuristic fallback filter for --no-llm mode #143 filtering_mode metadata on LLM-runtime-failure path: requires adding a state field + TypedDict change across 3 files — deferred as metadata-accuracy improvement.
• fix(static-runner): skip binary/PDF files and filter PE3 .env doc references #157 .svg in _BINARY_EXTENSIONS: code isn't on main yet (fix(static-runner): skip binary/PDF files and filter PE3 .env doc references #157 still open). Will be addressed on the fix(static-runner): skip binary/PDF files and filter PE3 .env doc references #157 branch or after merge.
• feat(cli): add --recursive flag for multi-skill directory scanning #141 remaining nits (dead finally: pass, per-skill cleanup path, typing, summary count): finally: pass already removed; others are minor style items.