fix: address code review feedback - docs, deprecation, consistency

[Copilot]

Addresses 9 review comments from PR #11 SecurityAgent implementation.

Documentation

• Remove incorrect "entropy analysis" claims in docstrings (lines 27, 436) - implementation uses regex + placeholder detection only
• Fix typo: "corregerse" → "corregirse"

Code Quality

• Add explanatory comments to empty except SyntaxError clauses - clarifies intentional silent failures on unparseable code
• Add missing shebang #!/bin/bash to fix_code_quality.sh

Bug Fixes

• Regex pattern inconsistency: Password pattern required 3+ chars but code filtered <8 chars. Updated pattern to {8,} to match enforcement
• Deprecation: Replace datetime.utcnow() with datetime.now(timezone.utc) for Python 3.12+ compatibility

# Before
detected_at: datetime = Field(default_factory=datetime.utcnow)

# After  
detected_at: datetime = Field(default_factory=lambda: datetime.now(timezone.utc))

Test Fixes

• Correct test comment: MD5 detection is MEDIUM severity, not LOW

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