simplify pkg/hooks: drop unused EventSpec abstraction

[dgageot]

Drops the EventSpec table from pkg/hooks/events.go (deleted) and
inlines the only two facts the executor actually used:

• failClosed(event) — true only for EventPreToolUse
• stdoutAsContext(event) — true for the six events whose runtime
  consumes Result.AdditionalContext

The ToolScoped field was unused, and the exported EventSpecs() /
EventSpec / StdoutPolicy types had no callers anywhere in the
workspace. All public extension points are preserved
(RegisterResponseShape, RegisterResponseSchema, ResponseShape,
NewModelFactory, …). Behaviour is identical for hook authors and for
the runtime; the diff is purely dead/over-engineered code removal plus
some doc trimming.

Net: -73 source lines, one fewer file, no behaviour change.