docs: add ACP and internal systems implementation guides

- add ACP user and developer docs covering setup, lifecycle, callbacks,
  permissions, tool rendering, and runtime behavior
- add developer guides for agent loop, provider runtime resolution,
  prompt assembly, context caching/compression, gateway internals,
  session storage, tools runtime, trajectories, and cron internals
- refresh architecture, quickstart, installation, CLI reference, and
  environments docs to link the new implementation pages and ACP support

website/docs/developer-guide/gateway-internals.md

@@ -0,0 +1,95 @@
+---
+sidebar_position: 7
+title: "Gateway Internals"
+description: "How the messaging gateway boots, authorizes users, routes sessions, and delivers messages"
+---
+
+# Gateway Internals
+
+The messaging gateway is the long-running process that connects Hermes to external platforms.
+
+Key files:
+
+- `gateway/run.py`
+- `gateway/config.py`
+- `gateway/session.py`
+- `gateway/delivery.py`
+- `gateway/pairing.py`
+- `gateway/channel_directory.py`
+- `gateway/hooks.py`
+- `gateway/mirror.py`
+- `gateway/platforms/*`
+
+## Core responsibilities
+
+The gateway process is responsible for:
+
+- loading configuration from `.env`, `config.yaml`, and `gateway.json`
+- starting platform adapters
+- authorizing users
+- routing incoming events to sessions
+- maintaining per-chat session continuity
+- dispatching messages to `AIAgent`
+- running cron ticks and background maintenance tasks
+- mirroring/proactively delivering output to configured channels
+
+## Config sources
+
+The gateway has a multi-source config model:
+
+- environment variables
+- `~/.hermes/gateway.json`
+- selected bridged values from `~/.hermes/config.yaml`
+
+## Session routing
+
+`gateway/session.py` and `GatewayRunner` cooperate to map incoming messages to active session IDs.
+
+Session keying can depend on:
+
+- platform
+- user/chat identity
+- thread/topic identity
+- special platform-specific routing behavior
+
+## Authorization layers
+
+The gateway can authorize through:
+
+- platform allowlists
+- gateway-wide allowlists
+- DM pairing flows
+- explicit allow-all settings
+
+Pairing support is implemented in `gateway/pairing.py`.
+
+## Delivery path
+
+Outgoing deliveries are handled by `gateway/delivery.py`, which knows how to:
+
+- deliver to a home channel
+- resolve explicit targets
+- mirror some remote deliveries back into local history/session tracking
+
+## Hooks
+
+Gateway events emit hook callbacks through `gateway/hooks.py`. Hooks are local trusted Python code and can observe or extend gateway lifecycle events.
+
+## Background maintenance
+
+The gateway also runs maintenance tasks such as:
+
+- cron ticking
+- cache refreshes
+- session expiry checks
+- proactive memory flush before reset/expiry
+
+## Honcho interaction
+
+When Honcho is enabled, the gateway can keep persistent Honcho managers aligned with session lifetimes and platform-specific session keys.
+
+## Related docs
+
+- [Session Storage](./session-storage.md)
+- [Cron Internals](./cron-internals.md)
+- [ACP Internals](./acp-internals.md)