feat(core): add LoopbackChannel for headless agent communication

[bug-ops]

Parent

Part of #581

Summary

Add LoopbackChannel implementing Channel trait for headless (non-interactive) agent communication. This channel bridges the A2A TaskProcessor with the agent loop via tokio mpsc channels.

Scope

Files: crates/zeph-core/src/channel.rs, crates/zeph-core/src/lib.rs

• LoopbackEvent enum: Chunk(String), Flush, FullMessage(String), Status(String), ToolOutput{...}
• LoopbackHandle (caller side): input_tx: Sender<ChannelMessage>, output_rx: Receiver<LoopbackEvent>
• LoopbackChannel (agent side): input_rx: Receiver<ChannelMessage>, output_tx: Sender<LoopbackEvent>
• Constructor: LoopbackChannel::pair(buffer: usize) -> (LoopbackChannel, LoopbackHandle)
• impl Channel for LoopbackChannel:
  • recv() → blocks on input_rx
  • send(text) → LoopbackEvent::FullMessage
  • send_chunk(chunk) → LoopbackEvent::Chunk
  • flush_chunks() → LoopbackEvent::Flush
  • send_status(text) → LoopbackEvent::Status
  • confirm() → auto-approve (headless, no interactive prompts)

Acceptance Criteria

• LoopbackChannel::pair() returns working channel + handle pair
• All Channel trait methods implemented
• Unit tests for send/recv round-trip and confirm auto-approve

[bug-ops]

Update (2026-02-20): alignment with recent refactors

• PR refactor(core): decompose agent module into focused submodules #675 decomposed agent module into submodules — LoopbackChannel should go in crates/zeph-core/src/channel.rs (unchanged, file still exists as-is)
• PR refactor: replace anyhow with typed errors in library crates #676 replaced anyhow with typed errors in library crates — LoopbackChannel errors must use thiserror (e.g. ChannelError::Closed, ChannelError::SendFailed), not anyhow
• PR refactor(channels): replace rustyline with crossterm for CLI input #615 replaced rustyline with crossterm for CLI — LoopbackChannel follows the same Channel trait; no impact from crossterm migration
• AnyChannel enum in src/main.rs now needs a Loopback variant (feature-gated behind daemon)