Define Channel trait and wrap CLI in it

[bug-ops]

Description

Abstract the I/O channel so agent works with CLI or Telegram interchangeably.

Parent: #5

Branch

feat/m4/channel-trait

Files

• crates/zeph-core/src/channel.rs — Channel trait, CliChannel impl
• crates/zeph-core/src/agent.rs — refactor to use Channel
• crates/zeph-core/src/lib.rs — add module

Acceptance Criteria

• Channel trait defined
• CliChannel wraps stdin/stdout
• Agent loop uses Channel trait (not direct stdin)
• Existing CLI behavior preserved

Depends On

#20 (Provider selection — M3 complete)