feat: add context memory layer + task-aware planner

[inspiringsource]

Key additions:

• Repository memory (.ai/memory.yaml) based on git co-change history
• Task-aware planning via aicontext plan "<task>"
• Blended ranking (structure + task priors + memory)
• Output includes:
  • read first
  • likely edit candidates
  • impacted files
  • likely tests

Goal:
Improve first-step navigation, reduce irrelevant exploration, and support better multi-file changes.

Notes:

• Deterministic, no embeddings or external services
• Lightweight and model-agnostic
• Tests passing (23/23)