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/session-storage.md

@@ -0,0 +1,66 @@
+---
+sidebar_position: 8
+title: "Session Storage"
+description: "How Hermes stores sessions in SQLite, maintains lineage, and exposes recall/search"
+---
+
+# Session Storage
+
+Hermes uses a SQLite-backed session store as the main source of truth for historical conversation state.
+
+Primary files:
+
+- `hermes_state.py`
+- `gateway/session.py`
+- `tools/session_search_tool.py`
+
+## Main database
+
+The primary store lives at:
+
+```text
+~/.hermes/state.db
+```
+
+It contains:
+
+- sessions
+- messages
+- metadata such as token counts and titles
+- lineage relationships
+- full-text search indexes
+
+## What is stored per session
+
+Examples of important session metadata:
+
+- session ID
+- source/platform
+- title
+- created/updated timestamps
+- token counts
+- tool call counts
+- stored system prompt snapshot
+- parent session ID after compression splits
+
+## Lineage
+
+When Hermes compresses a conversation, it can continue in a new session ID while preserving ancestry via `parent_session_id`.
+
+This means resuming/searching can follow session families instead of treating each compressed shard as unrelated.
+
+## Gateway vs CLI persistence
+
+- CLI uses the state DB directly for resume/history/search
+- gateway keeps active-session mappings and may also maintain additional platform transcript/state files
+- some legacy JSON/JSONL artifacts still exist for compatibility, but SQLite is the main historical store
+
+## Session search
+
+The `session_search` tool uses the session DB's search features to retrieve and summarize relevant past work.
+
+## Related docs
+
+- [Gateway Internals](./gateway-internals.md)
+- [Prompt Assembly](./prompt-assembly.md)
+- [Context Compression & Prompt Caching](./context-compression-and-caching.md)