- PluginContext.register_context_engine() lets plugins replace the
  built-in ContextCompressor with a custom ContextEngine implementation
- PluginManager stores the registered engine; only one allowed
- run_agent.py checks for a plugin engine at init before falling back
  to the default ContextCompressor
- reset_session_state() now calls engine.on_session_reset() instead of
  poking internal attributes directly
- ContextCompressor.on_session_reset() handles its own internals
  (_context_probed, _previous_summary, etc.)
- 19 new tests covering ABC contract, defaults, plugin slot registration,
  rejection of duplicates/non-engines, and compressor reset behavior
- All 34 existing compressor tests pass unchanged

- Inject engine tool schemas into agent tool surface after compressor init
- Call on_session_start() with session_id, hermes_home, platform, model
- Dispatch engine tool calls (lcm_grep, etc.) before regular tool handler
- 55/55 tests pass

…very, ABC completeness

Follow-up fixes for the context engine plugin slot (PR #5700):

- Enhance ContextEngine ABC: add threshold_percent, protect_first_n,
  protect_last_n as class attributes; complete update_model() default
  with threshold recalculation; clarify on_session_end() lifecycle docs
- Add ContextCompressor.update_model() override for model/provider/
  base_url/api_key updates
- Replace all direct compressor internal access in run_agent.py with
  ABC interface: switch_model(), fallback restore, context probing
  all use update_model() now; _context_probed guarded with getattr/
  hasattr for plugin engine compatibility
- Create plugins/context_engine/ directory with discovery module
  (mirrors plugins/memory/ pattern) — discover_context_engines(),
  load_context_engine()
- Add context.engine config key to DEFAULT_CONFIG (default: compressor)
- Config-driven engine selection in run_agent.__init__: checks config,
  then plugins/context_engine/<name>/, then general plugin system,
  falls back to built-in ContextCompressor
- Wire on_session_end() in shutdown_memory_provider() at real session
  boundaries (CLI exit, /reset, gateway expiry)

…egories

- Remove auto-activation: when context.engine is 'compressor' (default),
  plugin-registered engines are NOT used. Users must explicitly set
  context.engine to a plugin name to activate it.

- Add curses_radiolist() to curses_ui.py: single-select radio picker
  with keyboard nav + text fallback, matching curses_checklist pattern.

- Rewrite cmd_toggle() as composite plugins UI:
  Top section: general plugins with checkboxes (existing behavior)
  Bottom section: provider plugin categories (Memory Provider, Context Engine)
  with current selection shown inline. ENTER/SPACE on a category opens
  a radiolist sub-screen for single-select configuration.

- Add provider discovery helpers: _discover_memory_providers(),
  _discover_context_engines(), config read/save for memory.provider
  and context.engine.

- Add tests: radiolist non-TTY fallback, provider config save/load,
  discovery error handling, auto-activation removal verification.

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