Extends the TTS lease from #100912 (4e3feb8bbb) beyond built-in local
engines: when the configured tts.provider is user-declared, acquiring
the first lease and releasing the last one now reach it, so a
self-hosted TTS server can preload its model when read-aloud / voice
conversation turns on and unload when it turns off (Discord request).
- agent/tts_provider.py: TTSProvider gains concrete no-op warm() /
release() (not abstract — existing plugins are unaffected).
- tools/tts_tool.py: _signal_user_tts_provider() forwards the lease
hook; plugin providers get warm()/release(), command providers run
optional `warm_command` / `release_command` (config.yaml, under
tts.providers.<name>) through the existing _run_command_tts helper
on a daemon thread — best-effort, output discarded, failures at
debug. warm_tts_provider() and release_tts_provider() call it.
- tests/tools/test_tts_lifecycle_leases.py: fake plugin provider and
fake command provider observe warm/release through acquire/release
lease (both fail on main with action == "noop").
- docs: features/tts.md — lease section, command-provider optional
keys table, plugin optional hooks.
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