fix(hooks): include required hookEventName in hookSpecificOutput JSON

outputWithContext() emitted a hookSpecificOutput object without the
required hookEventName field, so Claude Code rejected the hook output
("hookSpecificOutput is missing required field hookEventName") and
dropped the context the SessionStart, UserPromptSubmit, and PostToolUse
hooks tried to inject. Thread the event name through the helper; each
caller now passes its own (SessionStart / UserPromptSubmit / PostToolUse).
Existing emission tests now assert hookEventName per event.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VmHCQjJHUyWwxGAVVjNLgp

CHANGELOG.md

@@ -2,6 +2,18 @@
 
 All notable changes to this project will be documented in this file.
 
+## [Unreleased]
+
+### Fixed
+
+- **Hooks emitted invalid JSON, so Claude Code dropped their injected
+  context.** `outputWithContext()` wrote a `hookSpecificOutput` object
+  without the required `hookEventName` field; the harness rejected it
+  ("hookSpecificOutput is missing required field hookEventName") and
+  discarded the context the SessionStart, UserPromptSubmit, and PostToolUse
+  hooks tried to inject. The helper now takes the event name and every
+  caller passes its own (`SessionStart` / `UserPromptSubmit` / `PostToolUse`).
+
 ## [1.2.0] — 2026-05-01
 
 Research-paper-driven detector update. Implements operational findings from