refactor(hooks): simplify package while preserving features

[dgageot]

Trim ~30% of non-test code in pkg/hooks while keeping every public API
and behavioral contract intact. All hooks tests, runtime hook-wiring
tests, go vet, and golangci-lint pass.

Changes

• Unify the Executor event lookup: remove the eventHooks{flat, matchers} split and the selectHooks helper. Every event now compiles
  to a single []matcher slice; flat events become one matcher with an
  empty pattern, so dispatch is one filter loop.
• Inline executeHooks + aggregateResults + dedup into Dispatch.
  The 5-return-value executeHook becomes runHook returning a
  hookResult. aggregate uses a clean switch over per-hook outcomes.
• Replace the contextEvents switch with a small
  map[EventType]bool to decide which events route plain stdout into
  AdditionalContext.
• Merge builtin.go into handler.go: registry, factories, and
  handlers now live in one file. HookTypeBuiltin moves into types.go
  alongside HookTypeCommand.
• Compact Config.IsEmpty, convertHooks/convertMatchers, and the
  builtins Register (single map iteration). Extract a small
  turnStartContext helper to remove duplicated Output boilerplate.
• Trim verbose multi-paragraph doc blocks across all files while
  keeping concise but informative godoc on public APIs.

Diff stats

7 files changed, 404 insertions(+), 766 deletions(-)

File	Before	After
types.go	314	199
executor.go	447	330
handler.go	189	200 (now also contains old builtin.go)
builtin.go	96	removed
config.go	63	53
builtins/builtins.go	132	97