feat: optional idle-leave-after-playback timeout

[seitzbg]

Summary

• Adds an optional, default-off IDLE_LEAVE_TIMEOUT (seconds). When playback ends — a video finishing on its own or !stop — and nothing new starts within the timeout, the bot leaves voice and posts a short note. Unset / empty / 0 keeps today's behavior (the bot stays put).
• Per-guild asyncio timer armed from the playback-cleanup (finally) blocks in cogs/stream.py (covers both the camera and go-live paths). A fire-time guard re-checks the bot is genuinely idle (stream_tasks/live_connections empty, vc not playing, still connected) before disconnecting, so it never cuts off active or resumed playback.
• Starting new playback re-arms (replacing any prior timer); !leave and auto-leave-on-empty cancel it. Timers are also cancelled on bot shutdown.

Implementation notes

• New helpers in cogs/stream.py: _idle_leave_timeout, _format_idle, schedule_idle_leave, cancel_idle_leave, _idle_leave_after, _idle_leave_now.
• SlopSoil gains idle_leave_tasks: dict[int, asyncio.Task] (cancelled in close()).
• cogs/voice.py cancels the timer on !leave and auto-leave-on-empty.
• Docs: IDLE_LEAVE_TIMEOUT added to .env.example and the README config table + features list.
• Design spec and implementation plan included under docs/superpowers/.

Test Plan

• pytest — full suite green (176 tests), including new tests/test_idle_leave.py (config parsing, duration formatting, schedule/re-arm/cancel, idle-guard no-ops, and !leave/auto-leave cancellation wiring).
• Manual: set IDLE_LEAVE_TIMEOUT=30, play a short clip, let it finish → bot leaves after ~30s and posts "left after 30 sec idle".
• Manual: !stop mid-clip → bot leaves after the timeout.
• Manual: start a new video before the timeout → bot stays; !leave before the timeout → no spurious idle message.
• Manual: unset/0 → bot never auto-leaves on idle (current behavior).

[seitzbg]

Superseded by the upstream PR against dev-topsoil/slopsoil: dev-topsoil#16 (clean, idle-leave-only, based on upstream main).