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>

pyproject.toml

@@ -17,6 +17,7 @@ dependencies = [
     "pydantic>=2.7",
     "pydantic-settings>=2.3",
     "omegaconf>=2.3",
+    "mautrix>=0.21.0",
 ]
 
 [project.optional-dependencies]
@@ -31,7 +32,7 @@ dev = [
 ]
 
 [project.scripts]
-steward = "steward.main:main"
+steward = "steward.main:run"
 
 [tool.setuptools.packages.find]
 where = ["."]