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/architecture.md

@@ -62,7 +62,8 @@ hermes-agent/
 │
 ├── agent/                    # Agent internals
 │   ├── prompt_builder.py     # System prompt assembly
-│   ├── context_compressor.py # Conversation compression algorithm
+│   ├── context_engine.py     # ContextEngine ABC (pluggable)
+│   ├── context_compressor.py # Default engine — lossy summarization
 │   ├── prompt_caching.py     # Anthropic prompt caching
 │   ├── auxiliary_client.py   # Auxiliary LLM for side tasks (vision, summarization)
 │   ├── model_metadata.py     # Model context lengths, token estimation
@@ -123,6 +124,7 @@ hermes-agent/
 ├── acp_adapter/              # ACP server (VS Code / Zed / JetBrains)
 ├── cron/                     # Scheduler (jobs.py, scheduler.py)
 ├── plugins/memory/           # Memory provider plugins
+├── plugins/context_engine/   # Context engine plugins
 ├── environments/             # RL training environments (Atropos)
 ├── skills/                   # Bundled skills (always available)
 ├── optional-skills/          # Official optional skills (install explicitly)
@@ -227,7 +229,7 @@ Long-running process with 14 platform adapters, unified session routing, user au
 
 ### Plugin System
 
-Three discovery sources: `~/.hermes/plugins/` (user), `.hermes/plugins/` (project), and pip entry points. Plugins register tools, hooks, and CLI commands through a context API. Memory providers are a specialized plugin type under `plugins/memory/`.
+Three discovery sources: `~/.hermes/plugins/` (user), `.hermes/plugins/` (project), and pip entry points. Plugins register tools, hooks, and CLI commands through a context API. Two specialized plugin types exist: memory providers (`plugins/memory/`) and context engines (`plugins/context_engine/`). Both are single-select — only one of each can be active at a time, configured via `hermes plugins` or `config.yaml`.
 
 → [Plugin Guide](/docs/guides/build-a-hermes-plugin), [Memory Provider Plugin](./memory-provider-plugin.md)