docs: bring hooks reference up to date with new events

[dgageot]

The hooks documentation was lagging behind the runtime. Brings it back in line with what the runtime actually dispatches today.

Doc changes (docs/configuration/hooks/index.md)

• Hook Types table: was titled "seven hook event types" while listing 11; now lists all 21 events the runtime dispatches. Added turn_start, before_llm_call, after_llm_call, on_error, on_max_iterations, on_agent_switch, on_session_resume, on_tool_approval_decision, before_compaction, after_compaction, plus a callout clarifying the difference between pre_compact and before_compaction.
• Input fields: replaced the wide matrix (covered 11 events, missing many fields) with a Common Fields + Per-Event Extra Fields layout. agent_name is no longer claimed as a common field (only stop, after_llm_call, and subagent_stop populate it). Adds the previously-undocumented fields: tool_error, last_user_message, from_agent / to_agent / agent_switch_kind, previous_max_iterations / new_max_iterations, approval_decision / approval_source, input_tokens / output_tokens / context_limit / compaction_reason / summary.
• Output: new Before-Compaction Specific Output section documenting hook_specific_output.summary; new Context-Contributing Events section clarifying transient vs persisted context. Fixed misleading "decision": "allow" example (the field is for blocking only).
• Plain Text Output: added turn_start to the list of context-contributing events.
• Per-event narrative subsections for turn_start, before/after_llm_call, before/after_compaction, on_agent_switch, on_session_resume, on_tool_approval_decision. Notification section now points readers at on_error / on_max_iterations.
• Removed an orphan </div> that was unbalancing the HTML between the LLM-Judge subsection and the CLI Flags section.

Code change (pkg/hooks/events.go)

Added explicit zero-value EventSpec entries for before_compaction, after_compaction, on_agent_switch, on_session_resume, on_tool_approval_decision. Behaviorally a no-op (eventSpec() already falls through to a zero-value EventSpec for unregistered events, which is the right default for these observational events — verified by TestPlainStdoutBecomesAdditionalContext), but it makes eventSpecs (and thus the EventSpecs() accessor) reflect the full set of events the runtime dispatches.

Validation

• mise lint — clean (golangci-lint, custom lint, go mod tidy)
• mise test — all packages pass