docs: context engine plugin system + unified hermes plugins UI

New page:
- developer-guide/context-engine-plugin.md — full guide for building
  context engine plugins (ABC contract, lifecycle, tools, registration)

Updated pages (11 files):
- plugins.md — plugin types table, composite UI documentation with
  screenshot-style example, provider plugin config format
- cli-commands.md — hermes plugins section rewritten for composite UI
  with provider plugin config keys documented
- context-compression-and-caching.md — new 'Pluggable Context Engine'
  section explaining the ABC, config-driven selection, resolution order
- configuration.md — new 'Context Engine' config section with examples
- architecture.md — context_engine.py and plugins/context_engine/ added
  to directory trees, plugin system description updated
- memory-provider-plugin.md — cross-reference tip to context engines
- memory-providers.md — hermes plugins as alternative setup path
- agent-loop.md — context_engine.py added to file reference table
- overview.md — plugins description expanded to cover all 3 types
- build-a-hermes-plugin.md — tip box linking to specialized plugin guides
- sidebars.ts — context-engine-plugin added to Extending category

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

@@ -111,10 +111,22 @@ Plugins can register callbacks for these lifecycle events. See the **[Event Hook
 | [`on_session_start`](/docs/user-guide/features/hooks#on_session_start) | New session created (first turn only) |
 | [`on_session_end`](/docs/user-guide/features/hooks#on_session_end) | End of every `run_conversation` call + CLI exit handler |
 
+## Plugin types
+
+Hermes has three kinds of plugins:
+
+| Type | What it does | Selection | Location |
+|------|-------------|-----------|----------|
+| **General plugins** | Add tools, hooks, CLI commands | Multi-select (enable/disable) | `~/.hermes/plugins/` |
+| **Memory providers** | Replace or augment built-in memory | Single-select (one active) | `plugins/memory/` |
+| **Context engines** | Replace the built-in context compressor | Single-select (one active) | `plugins/context_engine/` |
+
+Memory providers and context engines are **provider plugins** — only one of each type can be active at a time. General plugins can be enabled in any combination.
+
 ## Managing plugins
 
 ```bash
-hermes plugins                  # interactive toggle UI — enable/disable with checkboxes
+hermes plugins                  # unified interactive UI
 hermes plugins list             # table view with enabled/disabled status
 hermes plugins install user/repo  # install from Git
 hermes plugins update my-plugin   # pull latest
@@ -123,7 +135,37 @@ hermes plugins enable my-plugin   # re-enable a disabled plugin
 hermes plugins disable my-plugin  # disable without removing
 ```
 
-Running `hermes plugins` with no arguments launches an interactive curses checklist (same UI as `hermes tools`) where you can toggle plugins on/off with arrow keys and space.
+### Interactive UI
+
+Running `hermes plugins` with no arguments opens a composite interactive screen:
+
+```
+Plugins
+  ↑↓ navigate  SPACE toggle  ENTER configure/confirm  ESC done
+
+  General Plugins
+ → [✓] my-tool-plugin — Custom search tool
+   [ ] webhook-notifier — Event hooks
+
+  Provider Plugins
+     Memory Provider          ▸ honcho
+     Context Engine           ▸ compressor
+```
+
+- **General Plugins section** — checkboxes, toggle with SPACE
+- **Provider Plugins section** — shows current selection. Press ENTER to drill into a radio picker where you choose one active provider.
+
+Provider plugin selections are saved to `config.yaml`:
+
+```yaml
+memory:
+  provider: "honcho"      # empty string = built-in only
+
+context:
+  engine: "compressor"    # default built-in compressor
+```
+
+### Disabling general plugins
 
 Disabled plugins remain installed but are skipped during loading. The disabled list is stored in `config.yaml` under `plugins.disabled`: