docs: add documentation for Tier 2 features

- README: webhooks, heartbeat, vector memory, Docker sections with config examples
- README: updated features list, architecture tree, environment variables
- CHANGELOG: entries for all 4 Tier 2 features

CHANGELOG.md

@@ -6,6 +6,23 @@ All notable changes to Flynn are documented in this file.
 
 ### Added
 
+- **Inbound Webhooks** -- HTTP endpoints (`POST /webhooks/:name`) that trigger agent
+  processing. Config-driven with per-webhook HMAC signature verification, message
+  template rendering (`{{body}}`, `{{json.field}}`), and output channel routing.
+  Bypasses gateway token auth in favour of per-webhook secrets.
+- **Heartbeat Monitor** -- Periodic health check system with 5 configurable checks:
+  gateway responsiveness, model router status, channel adapter connectivity, memory
+  store accessibility, and disk space. Sends failure notifications after a configurable
+  threshold and recovery notifications when health restores.
+- **Vector Memory Search** -- Hybrid keyword + semantic search for the memory system.
+  Provider-agnostic embeddings (OpenAI, Gemini, Ollama, llama.cpp) stored in SQLite
+  with brute-force cosine similarity. Background indexer processes dirty namespaces
+  every 30s. Configurable hybrid weighting (0.0 = keyword only, 1.0 = vector only).
+  Graceful fallback to keyword search when embeddings are unavailable.
+- **Docker Deployment** -- Multi-stage Dockerfile (node:22-alpine build + runtime),
+  `.dockerignore`, and `docker-compose.yml`. Native dependency handling for
+  `better-sqlite3`. `FLYNN_DATA_DIR` environment variable for configurable data
+  directory in container deployments.
 - **Agent Tools: sessions.\*** -- 4 new agent-callable tools (`sessions.list`,
   `sessions.history`, `sessions.create`, `sessions.delete`) wrapping SessionManager
   for runtime session management by the AI agent