docs: unify hooks documentation — add plugin hooks to hooks page, add session:end event

The hooks page only documented gateway event hooks (HOOK.yaml system).
The plugins page listed plugin hooks (pre_tool_call, etc.) that weren't
referenced from the hooks page, which was confusing.

Changes:
- hooks.md: Add overview table showing both hook systems
- hooks.md: Add Plugin Hooks section with available hooks, callback
  signatures, and example
- hooks.md: Add missing session:end gateway event (emitted but undocumented)
- hooks.md: Mark pre_llm_call, post_llm_call, on_session_start,
  on_session_end as planned (defined in VALID_HOOKS but not yet invoked)
- hooks.md: Update info box to cross-reference plugin hooks
- hooks.md: Fix heading hierarchy (gateway content as subsections)
- plugins.md: Add cross-reference to hooks page for full details
- plugins.md: Mark planned hooks as (planned)

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

@@ -46,14 +46,16 @@ Project-local plugins under `./.hermes/plugins/` are disabled by default. Enable
 
 ## Available hooks
 
+Plugins can register callbacks for these lifecycle events. See the **[Event Hooks page](/docs/user-guide/features/hooks#plugin-hooks)** for full details, callback signatures, and examples.
+
 | Hook | Fires when |
 |------|-----------|
 | `pre_tool_call` | Before any tool executes |
 | `post_tool_call` | After any tool returns |
-| `pre_llm_call` | Before LLM API request |
-| `post_llm_call` | After LLM API response |
-| `on_session_start` | Session begins |
-| `on_session_end` | Session ends |
+| `pre_llm_call` | Before LLM API request *(planned)* |
+| `post_llm_call` | After LLM API response *(planned)* |
+| `on_session_start` | Session begins *(planned)* |
+| `on_session_end` | Session ends *(planned)* |
 
 ## Slash commands