feat: enable Matrix E2EE with cross-signing verification on bot

Add end-to-end encryption support to the Matrix bot using the matrix-sdk
crypto features. The bot now:
- Enables E2EE on the Matrix client with cross-signing bootstrapping
- Auto-verifies its own cross-signing identity on startup
- Handles key verification requests from other users automatically
- Sends encrypted messages in E2EE-enabled rooms
- Adds MATRIX_STORE_PATH config for persistent crypto store

Squash merge of feature/story-194_story_enable_matrix_e2ee_with_cross_signing_verification_on_bot

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/main.rs

@@ -1,3 +1,7 @@
+// matrix-sdk-crypto's deeply nested types require a higher recursion limit
+// when the `e2e-encryption` feature is enabled.
+#![recursion_limit = "256"]
+
 mod agent_log;
 mod agents;
 mod config;