Implement agent hooks (prepare/filter/retry)

[endymion]

Problem

The spec describes agent lifecycle hooks (prepare, filter, response retry config), but they are not implemented.

Why it matters

• Users need per-agent customization (pre-turn context, token budgeting, retry behavior) without forking core code.

Where this shows up

• TECHNICAL_DEBT.md → “Agent Hooks” (Category 3).

Proposed fix

• Implement agent config fields:
  • prepare: callable run before each turn; output becomes {prepared.*} (when/if that namespace is implemented)
  • filter: token budget / message filtering policy
  • response: retry policy (retries, retry_delay, etc.)
• If {prepared.*} is not implemented yet, define a minimal path (e.g., only allow prepare to mutate session/context).

Acceptance criteria

• Hooks are parsed, validated, and executed according to spec (or spec is updated to match final design).
• Tests cover hook execution order and retry behavior.