docs: deep quality pass — expand 10 thin pages, fix specific issues (#4134)

Developer guide stubs expanded to full documentation:
- trajectory-format.md: 56→233 lines (JSONL format, ShareGPT example,
  normalization rules, reasoning markup, replay code)
- session-storage.md: 66→388 lines (SQLite schema, migration table,
  FTS5 search syntax, lineage queries, Python API examples)
- context-compression-and-caching.md: 72→321 lines (dual compression
  system, config defaults, 4-phase algorithm, before/after example,
  prompt caching mechanics, cache-aware patterns)
- tools-runtime.md: 65→246 lines (registry API, dispatch flow,
  availability checking, error wrapping, approval flow)
- prompt-assembly.md: 89→246 lines (concrete assembled prompt example,
  SOUL.md injection, context file discovery table)

User-facing pages expanded:
- docker.md: 62→224 lines (volumes, env forwarding, docker-compose,
  resource limits, troubleshooting)
- updating.md: 79→167 lines (update behavior, version checking,
  rollback instructions, Nix users)
- skins.md: 80→206 lines (all color/spinner/branding keys, built-in
  skin descriptions, full custom skin YAML template)

Hub pages improved:
- integrations/index.md: 25→82 lines (web search backends table,
  TTS/browser providers, quick config example)
- features/overview.md: added Integrations section with 6 missing links

Specific fixes:
- configuration.md: removed duplicate Gateway Streaming section
- mcp.md: removed internal "PR work" language
- plugins.md: added inline minimal plugin example (self-contained)

13 files changed, ~1700 lines added. Docusaurus build verified clean.

website/docs/user-guide/features/plugins.md

@@ -25,6 +25,56 @@ Drop a directory into `~/.hermes/plugins/` with a `plugin.yaml` and Python code:
 
 Start Hermes — your tools appear alongside built-in tools. The model can call them immediately.
 
+### Minimal working example
+
+Here is a complete plugin that adds a `hello_world` tool and logs every tool call via a hook.
+
+**`~/.hermes/plugins/hello-world/plugin.yaml`**
+
+```yaml
+name: hello-world
+version: "1.0"
+description: A minimal example plugin
+```
+
+**`~/.hermes/plugins/hello-world/__init__.py`**
+
+```python
+"""Minimal Hermes plugin — registers a tool and a hook."""
+
+
+def register(ctx):
+    # --- Tool: hello_world ---
+    schema = {
+        "name": "hello_world",
+        "description": "Returns a friendly greeting for the given name.",
+        "parameters": {
+            "type": "object",
+            "properties": {
+                "name": {
+                    "type": "string",
+                    "description": "Name to greet",
+                }
+            },
+            "required": ["name"],
+        },
+    }
+
+    def handle_hello(params):
+        name = params.get("name", "World")
+        return f"Hello, {name}! 👋  (from the hello-world plugin)"
+
+    ctx.register_tool("hello_world", schema, handle_hello)
+
+    # --- Hook: log every tool call ---
+    def on_tool_call(tool_name, params, result):
+        print(f"[hello-world] tool called: {tool_name}")
+
+    ctx.register_hook("post_tool_call", on_tool_call)
+```
+
+Drop both files into `~/.hermes/plugins/hello-world/`, restart Hermes, and the model can immediately call `hello_world`. The hook prints a log line after every tool invocation.
+
 Project-local plugins under `./.hermes/plugins/` are disabled by default. Enable them only for trusted repositories by setting `HERMES_ENABLE_PROJECT_PLUGINS=true` before starting Hermes.
 
 ## What plugins can do