Implement cross-session decision tracking

[Jmeg8r]

Build the mechanism to extract, store, and surface decisions made across sessions. This allows Cortex to remember architectural choices, tool preferences, and design rationale.

Acceptance Criteria

• Decision extraction from session transcripts
• Persistent storage for decision records
• Decision retrieval API for briefing generation
• Conflict detection when new decisions contradict old ones
• Tests covering decision lifecycle