F07-T03: Add DEBUG logging for alias resolution

[leocamello]

Description

Add structured logging at DEBUG level for alias resolution, showing each hop and the final resolved model.

Expected Log Output

DEBUG routing: Resolved alias from="gpt-4" to="llama-large" depth=1
DEBUG routing: Resolved alias from="llama-large" to="llama3:70b" depth=2
DEBUG routing: Alias resolution complete original="gpt-4" resolved="llama3:70b" chain_depth=2

Acceptance Criteria

• Each alias hop logged at DEBUG level
• Final resolution logged with chain depth
• No logging when no alias used

Files

• src/routing/mod.rs