Make the 12 system workers spawn-safe: entry classes replace AgentRuntime closures

Group B of the idea-5 spawn-safety design. Every AgentRuntime._register_*
control worker was an async-def closure ('<locals>' qualname — unpicklable
under 'spawn'): guardrails (combined + single), stop_when, gate, callback,
termination, check_transfer, transfer (hybrid + swarm + unreachable),
router, handoff_check, process_selection. Each becomes a picklable
module-level entry class with async __call__ and plain-data attrs:

- User callables (guardrail funcs, stop_when/gate/router fns, legacy
  callbacks) travel as FunctionRef when module-level, raw otherwise —
  raw only survives fork, and every registration site now runs
  probe_spawn_safety (group "system"), so lambdas fail fast at
  registration with the offender named (design §7 policy).
- CallbackEntry carries handlers (by value) + legacy fn (by ref) and
  rebuilds the _chain_callbacks_for_position chain per call — the chain
  closure itself was unpicklable. _register_callback_worker's signature
  changes accordingly (single caller updated).
- HandoffCheckEntry keeps blocked_counts as instance state (per worker
  process — the same semantics as the closure cell it replaces);
  HandoffConditions travel by value.
- TerminationEntry carries the condition object by value; transfer
  workers become stateless TransferNoopEntry/TransferUnreachableEntry.

Tests: async entry across a real spawn boundary, handoff decision +
pickle round-trips, callback chain rebuild, fail-fast policy (lambda →
SpawnSafetyError at registration). tests/unit: 2518 passed, 1 skipped.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: Kowser