feat(prime): continual harness, autonomous budgets, family messaging - #248

Three adoptions from Prime Agent's RLM agent.

Continual harness (internal/intelligence/harness):
- Versioned, evidence-backed refinement store for supplemental agent state
  (prompts, memories, skill descriptions, subagent specs).
- Refine bumps entry version and records a refinement event
  (trigger -> changes -> evidence -> outcome); every change is auditable
  and reversible via snapshot-based rollback. Never rewrites the
  immutable base system prompt. Durable JSON persistence.

Bounded autonomous budgets (internal/engine/autonomous_budget.go):
- Tracks turns/tokens/time/continuations within configured limits and a
  status machine that reports WHY a run stopped: budget_limited:<which>
  vs gate_passed vs error (a passed gate checks only what it verifies;
  reaching a limit does not imply success). First budget hit wins and
  terminal reasons stick.

Agent family messaging (internal/multiagent/family_messaging.go):
- Direct inter-agent messages scoped to a family graph (parent / sibling
  / child) rather than global, with per-agent pending caps and token-
  bucket rate limiting to prevent flooding.

Verification: new suites green (harness 8, autonomous 8, family 7);
engine + multiagent suites pass; golangci-lint 0 issues; gofmt clean.