[idea] Per-agent sound files (done_path / request_path for each agent)

[antenando]

I run Claude Code, Codex and Gemini in panes next to each other. When a sound plays, I cannot tell which agent finished or which agent needs me without looking at the screen.

Herdr already knows which agent is in each pane, and [ui.sound.agents] already lets me set each agent to on, off or default. But the sound files are global: one done_path and one request_path for all agents.

Proposal

Let an agent entry be a table as well as the current enum, with its own sound paths:

[ui.sound]
enabled = true
done_path = "sounds/done.mp3"
request_path = "sounds/request.mp3"

[ui.sound.agents]
gemini = "off"            # the current enum form still works

[ui.sound.agents.claude]
done_path = "sounds/claude-done.mp3"
request_path = "sounds/claude-request.mp3"

[ui.sound.agents.codex]
done_path = "sounds/codex-done.mp3"   # request_path not set: use the global one

Rules:

• An agent path that is not set falls back to the global done_path / request_path, then to the built-in sound.
• Relative paths resolve from the config file's directory, as the global paths do now.
• The string forms "on", "off" and "default" keep working, so current configs do not change.
• An optional enabled = false in the table can replace "off", if you prefer one form.

Why in Herdr, not in a plugin

I can do this now with a plugin hook on pane.agent_status_changed, but then each user must:

• find a player that works on macOS, Linux and Windows (Herdr already solved this),
• set the agents to off in [ui.sound.agents] so two sounds do not play,
• repeat the focus rules that Herdr uses to decide when to play a sound.

Herdr already has the agent name, the player and the focus rules at the point where it plays the sound, so this looks like a small change there.

Related: #4995 (sound packs plugin), #1719 (sound volume), #4662 (sounds in the focused workspace).