Epic: Unify agent runner — stdin-driven persistent mode in Omni/Agent

Context

Agentd currently uses two different agent runners:

• Omni/Agent for one-shot runs (shebang interpreter, emits Trace.Event JSONL, supports agentd replay/debug)
• pi --mode rpc for persistent runs (stdin RPC loop, no Trace.Event emission, no replay/debug support)

This split means persistent agents can't use agentd's replay/debug tooling, and we maintain two runners. The goal is to make Omni/Agent the sole runner for both modes, bringing its Trace.Event integration to persistent workflows, while keeping it Unix-pure (no --mode flags, no RPC JSON schema).

Design: Unix-native stdin-driven mode

Instead of pi's JSON command schema, agent becomes a well-behaved Unix program:

• agent script.md → current behavior, shebang-compatible, exits when loop finishes
• agent with no script → reads prompts from stdin delimited by null bytes (\0); each prompt runs the loop, then blocks reading the next prompt; EOF → clean exit
• SIGINT → cancel current iteration (unwinds via SteeringCancel in Op loop)
• SIGTERM → finish current iteration and exit
• No set_model / shutdown commands — use the signal vocabulary Unix already provides
• Steering file goes away (replaced by writing to stdin)

Null-byte delimiter is chosen because it's binary-safe, unambiguous, trivially implemented, and matches xargs -0 / find -print0 conventions.

Why this is good

• Shebang case is completely unchanged
• One binary, one loop, two prompt sources (argv script or stdin)
• Agentd becomes trivial: spawn agent, hold stdin pipe, write null-terminated prompts, read Trace.Event JSONL from stdout
• Cancel is just SIGINT — same as every Unix REPL
• Replay/debug work for persistent agents because they emit the same Trace.Event stream

Subtasks

1. Add stdin prompt-reading mode to Omni/Agent (null-byte delimited) 2. Signal handling: SIGINT → cancel current iteration, SIGTERM → graceful exit 3. Swap Daemon.hs persistent-agent spawner from pi to agent 4. Remove pi-specific code paths and the steering file mechanism once cutover is verified