Implement JSONL RPC envelope for agent-orchestrator communication

Context

ADF currently communicates with agents via unstructured stdout/stderr pipes. This causes:

• SIGPIPE kills when stderr pipe is dropped
• Fragile text parsing for cost/session/status extraction
• No message framing, idempotency, or error typing

Proposal

Implement a JSONL envelope protocol inspired by Piwork's taskd RPC spec (docs/runtime-taskd-rpc-spec.md).

Envelope Format

Agent -> Orchestrator (event):

{"type": "event", "event": "cost_update", "agent": "security-sentinel", "ts": "...", "payload": {...}}

Orchestrator -> Agent (command, future):

{"id": "req_01", "type": "pause", "payload": {"reason": "budget_exceeded"}}

Key Properties

• Strict envelope: {type, event/id, payload} (payload required even if empty)
• Idempotency: duplicate request IDs with same payload produce no duplicate effects
• Typed error codes with retryable flag
• State recovery: runtime_get_state equivalent after reconnect

Integration

• For Claude agents: wrap --output-format stream-json events in ADF envelope
• For Codex agents: gracefully degrade to unstructured mode
• New agents: require envelope support as spawn prerequisite

Benefits

• Foundation for cost tracking (#638), session persistence (#639), activity log (#641)
• Eliminates SIGPIPE bug (structured framing detects incomplete messages)
• Enables future Agent SDK migration

References

• Piwork taskd RPC spec: https://github.com/ferologics/Piwork/blob/main/docs/runtime-taskd-rpc-spec.md
• Plan: cto-executive-system/plans/adf-architecture-improvements.md (item 1.3)
• Blocks: #638, #639, #641

Start by reading docs/runtime-taskd-rpc-spec.md and item 1.3 in cto-executive-system/plans/adf-architecture-improvements.md, then inspect the Claude stream-json and Codex agent communication paths. Done means the proposed envelope, idempotency, typed errors, state recovery, and graceful Codex fallback are implemented across the listed integrations.