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

__init__.py

@@ -0,0 +1,59 @@
+"""
+hermes-piper-plugin (piper)
+
+Overrides Hermes' default TTS provider from Edge to Piper at plugin load
+time. Uses runtime monkey-patching — zero modifications to the Hermes repo.
+"""
+
+from __future__ import annotations
+
+import logging
+
+logger = logging.getLogger(__name__)
+
+_PATCHED = False
+
+
+def _apply_piper_patch() -> None:
+    """Replace Edge default with Piper in TTS tool's provider selection.
+
+    Python module-level constants are mutable — ``DEFAULT_PROVIDER`` is a
+    string that ``_get_provider()`` reads at call time (not import time).
+    Changing it after import is safe and immediately effective.
+    """
+    global _PATCHED
+    if _PATCHED:
+        return
+
+    try:
+        import tools.tts_tool
+
+        old_default = getattr(tools.tts_tool, "DEFAULT_PROVIDER", "edge")
+        tools.tts_tool.DEFAULT_PROVIDER = "piper"
+
+        logger.info("piper-plugin: DEFAULT_PROVIDER changed from '%s' → 'piper'", old_default)
+        _PATCHED = True
+    except ImportError:
+        logger.warning("piper-plugin: tools.tts_tool not importable — patch deferred")
+    except Exception as exc:
+        logger.warning("piper-plugin: failed to patch DEFAULT_PROVIDER: %s", exc)
+
+
+def _on_session_start(**kw):
+    """Log Piper default status at session start (idempotent)."""
+    if not _PATCHED:
+        _apply_piper_patch()
+
+    try:
+        import tools.tts_tool
+        current = getattr(tools.tts_tool, "DEFAULT_PROVIDER", "?")
+        logger.info("piper-plugin: TTS default provider = %s (patched=%s)", current, _PATCHED)
+    except Exception:
+        pass
+
+
+def register(ctx) -> None:
+    """Register Piper default override."""
+    _apply_piper_patch()
+    ctx.register_hook("on_session_start", _on_session_start)
+    logger.info("piper-plugin: registered ✓ (TTS default → piper)")