Task session management for Arbiter.
Provides task sessions with call budgets, tool whitelisting, time limits, and TTL-based cleanup. Sessions track what an agent is authorized to do and enforce those bounds per-request.
The [StorageBackedSessionStore] provides persistent sessions through
an in-memory write-through cache backed by any SessionStore storage
implementation (REQ-001, REQ-007).
Behavioral anomaly detection lives in the arbiter-behavior crate,
which provides tiered intent classification (read/write/admin).