audio: CLI audio file input support

[bug-ops]

Parent: #520
Depends on: #521, #522

Context

Allow CLI users to send audio files as input via a command or flag.

Design

# Inline audio file
zeph --audio recording.mp3

# Or as a CLI command during conversation
/audio path/to/file.wav

Implementation

• Add `--audio ` CLI flag (clap)
• Add `/audio ` in-session command
• Read file, validate format, create `Attachment::Audio`
• Route through normal STT pipeline

Acceptance criteria

• `--audio` flag for single-shot mode
• `/audio` command during interactive session
• Supported formats: mp3, wav, ogg, flac, m4a, webm
• Clear error for unsupported formats