initial: Piper TTS default plugin

Runtime monkey-patch that changes tools.tts_tool.DEFAULT_PROVIDER from
'edge' to 'piper' at plugin load time. Zero fork needed.

- plugin.yaml with requires_pip: piper-tts
- __init__.py with _apply_piper_patch() + on_session_start hook
- README with installation and safety notes

plugin.yaml

@@ -0,0 +1,11 @@
+name: piper
+version: 1.0.0
+description: >
+  Makes Piper TTS the default provider instead of Edge. Zero fork needed —
+  uses runtime monkey-patching to set DEFAULT_PROVIDER = "piper" at plugin
+  load time. Survives Hermes upgrades without merge conflicts.
+author: "@gortium"
+hooks:
+  - on_session_start
+requires_pip:
+  - piper-tts