Implement JSONL RPC envelope for agent-orchestrator communication

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.

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