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/guides/build-a-hermes-plugin.md

@@ -547,6 +547,12 @@ After registration, users can run `hermes my-plugin status`, `hermes my-plugin c
 
 **Active-provider gating:** Memory plugin CLI commands only appear when their provider is the active `memory.provider` in config. If a user hasn't set up your provider, your CLI commands won't clutter the help output.
 
+:::tip
+This guide covers **general plugins** (tools, hooks, CLI commands). For specialized plugin types, see:
+- [Memory Provider Plugins](/docs/developer-guide/memory-provider-plugin) — cross-session knowledge backends
+- [Context Engine Plugins](/docs/developer-guide/context-engine-plugin) — alternative context management strategies
+:::
+
 ### Distribute via pip
 
 For sharing plugins publicly, add an entry point to your Python package: