feat(perf): regex prefilter — skip files before tree-sitter parse (closes #56 phase-1)

[Wolfvin]

Summary

Implements issue #56: regex prefilter to skip files before tree-sitter parse.

What changed

• New: scripts/prefilter.py
• Modified: scripts/commands/scan.py (integrate prefilter + --no-prefilter flag)
• New: tests for no-false-negatives guarantee

Verification

• Tests: 917 passed, 87 skipped (1 pre-existing main-branch failure in test_hybrid_engine unrelated to this PR)
• Scan with vs without prefilter: same findings (no false negatives verified end-to-end)
• Prefilter is conservative: returns None (no-op) when no rules loaded, returns True on any read error
• Verbose output matches spec: Prefilter: 1240 files checked, 387 passed, 853 skipped (68%) in 0.3s

Closes #56

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[sonarqubecloud]

Quality Gate failed

Failed conditions
C Reliability Rating on New Code (required ≥ A)

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