refactor: restrict Agent field visibility with accessor methods

[bug-ops]

Epic: #1977

All 21 Agent state fields are pub(super), meaning any of 30+ files in agent/ can access any field. Add accessor methods and change fields to pub(self) where possible.

Start with the most-accessed fields:

• memory_state -> memory_state(), memory_state_mut()
• skill_state -> skill_state(), skill_state_mut()
• context_manager -> context_manager(), context_manager_mut()

Acceptance Criteria:

• At least 5 fields accessed via methods only
• No behavioral changes
• All tests pass

Complexity: Large | Effort: 6h
Ref: ABS-04