refactor: overhaul hook system with dynamic types and convention-based registration

[htcarr3]

What

Replaces the rigid per-event hook system with a dynamic, convention-based architecture. Adding a new CLI hook event is now one string in an array instead of changes to three separate files.

Why

Every new hook event required updating HOOK_EVENTS, EVENT_MAP, and a define_input declaration — 23 events × 3 locations. The typed input classes (PreToolUseInput, etc.) weren't even used in the dispatch path. This refactor eliminates that maintenance burden and gives each type a clear reason to exist.

How

Hook base class — replaces HookMatcher. Owns matching (matches?), CLI serialization (to_config), and callback dispatch (dispatch). event_name is derived from the class name by convention (PreToolUseHook → "PreToolUse"). Subclasses are generated per event by HookRegistry.register.

HookInput — single dynamic class replacing 23 static *Input classes. Base fields (session_id, cwd, etc.) are first-class readers; event-specific fields use method_missing. Frozen at construction.

HookRegistry — Enumerable collection that persists as the data structure in Options#hooks (no more compile-to-hash step). register("PreToolUse") generates both a PreToolUseHook class and a before_tool_use DSL method. wrap normalizes any input type. from_hash builds from raw hashes.

Hook#dispatch — wires HookInput and HookContext into the actual callback dispatch path (previously callbacks received raw hashes).

Key changes

• -648 lines net (23 files, 780 additions, 1428 deletions)
• Hook, HookInput, HookContext, HookRegistry in hooks/ directory
• Hook::CallbackEntry replaces plain hash in callback registry
• build_hooks_config reduced from 20 lines to 7
• All docs, CHANGELOG, SPEC, and architecture guide updated