Implement aicheck CLI for AI content detection (#1)

Add a Rust CLI tool (aic) that detects AI-generated content via three provenance layers:
- C2PA manifest reading (HIGH confidence): digitalSourceType and claim_generator
- XMP/IPTC metadata parsing (MEDIUM confidence): DigitalSourceType, AISystemUsed, CreatorTool
- EXIF heuristics (LOW confidence): Software tag matching and camera metadata absence

Includes batch processing, JSON/human/quiet output modes, and metadata info dump.
Supports images, video, audio, and PDF formats.

Co-authored-by: Claude Haiku 4.5 <noreply@anthropic.com>

Author: MatrixA

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