feat: add Matrix appservice bot and platform-agnostic conversation core

Introduce a shared ConversationService (steward/bot/core.py) that owns the
LLM call, history, knowledge-base search, and thread-memory keying behind a
normalized ThreadKey, so both Telegram and Matrix drive the same pipeline.

- Add steward/bot/matrix.py: a mautrix-python appservice bot that receives
  Synapse transactions and replies via the client-server API.
- Refactor telegram.py handlers into thin wrappers over ConversationService.
- Generalize ThreadMemoryStore/ThreadSummary to platform-scoped keys with
  legacy chat_id:thread_id migration.
- Add a matrix config section (homeserver, tokens, room/user allowlists).
- Rewrite main.py as async, starting Telegram and/or Matrix on one event loop.
- Add mautrix>=0.21.0 dependency.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

steward/bot/thread_key.py

@@ -0,0 +1,24 @@
+"""Normalised conversation identity shared across platforms and storage."""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+
+@dataclass(frozen=True, slots=True)
+class ThreadKey:
+    """Normalised identity of a conversation scope across platforms.
+
+    ``platform`` is ``"telegram"`` or ``"matrix"``. ``scope`` is the chat/room/user
+    identifier as a string. ``thread`` is an optional sub-thread identifier.
+    """
+
+    platform: str
+    scope: str
+    thread: str | None = None
+
+    def __str__(self) -> str:
+        parts = [self.platform, self.scope]
+        if self.thread:
+            parts.append(self.thread)
+        return ":".join(parts)