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/developer-guide/agent-loop.md

@@ -226,7 +226,8 @@ After each turn:
 |------|---------|
 | `run_agent.py` | AIAgent class — the complete agent loop (~9,200 lines) |
 | `agent/prompt_builder.py` | System prompt assembly from memory, skills, context files, personality |
-| `agent/context_compressor.py` | Conversation compression algorithm |
+| `agent/context_engine.py` | ContextEngine ABC — pluggable context management |
+| `agent/context_compressor.py` | Default engine — lossy summarization algorithm |
 | `agent/prompt_caching.py` | Anthropic prompt caching markers and cache metrics |
 | `agent/auxiliary_client.py` | Auxiliary LLM client for side tasks (vision, summarization) |
 | `model_tools.py` | Tool schema collection, `handle_function_call()` dispatch |