fix(agents): correct user-invocable typo; document exact lh run flags

- All agent profiles used the misspelled 'user-invokable' property, which
  Copilot CLI silently ignores, so internal-only agents (scout, splitter,
  builder, verifier, integrator, scribe) were never actually hidden from
  the /agent picker. Renamed to the documented 'user-invocable'.
- conductor/architect instructions said only 'Run lh run event' with no
  flags, forcing the model to trial-and-error discover --type/--status/
  etc. Documented the exact lh run start/event/end CLI syntax inline.

.github/agents/scribe.agent.md

@@ -3,7 +3,7 @@ name: Scribe
 description: Continuously writes non-blocking journal, ADR, living spec, and convention updates from pipeline events.
 model: claude-haiku-4.5
 tools: [read, search, edit, execute]
-user-invokable: false
+user-invocable: false
 ---
 
 # Scribe