UserPromptSubmit hooks never fire on goal-continuation turns (origin kind guard)

[mikkihugo]

Component: agent-core-v2 external hooks + goal feature

Observed (verified on latest main 7f5debf, also present in released 0.41.0):

• packages/agent-core-v2/src/features/externalHooks/agent/agentExternalHooksService.ts:341:
  if ((ctx.promptMessage.origin ?? USER_PROMPT_ORIGIN).kind !== 'user') return false;
  so the UserPromptSubmit hook runs ONLY when the prompt origin kind is user.
• Goal continuations are submitted with a different origin —
  packages/agent-core-v2/src/features/goal/goalService.ts:101:
  { kind: 'system_trigger', name: 'goal_continuation' }.
• Therefore during goal mode every auto-continuation turn skips the UserPromptSubmit hook
  entirely, however long the goal runs.

Impact: hooks configured on UserPromptSubmit (mailbox sweeps, session-facts injection,
input gating, metrics) silently never fire for the whole duration of a goal session, which is
exactly the mode where the machine is meant to work unattended. There is no other hook event
that fires per goal turn (PreToolUse/PostToolUse fire only when tools run; Stop/
PreCompact are turn/compaction-adjacent and do not carry prompt input).

Suggested fix (any of):

1. Fire UserPromptSubmit for system_trigger origins whose name is goal_continuation
   (and document the matcher value), or
2. Add a dedicated GoalContinuation hook event so consumers opt in explicitly, or
3. Document this exclusion in the hooks reference so consumers can choose PostToolUse
   instead.

Test seam exists: the hook service already subscribes to PromptQueued and TurnStarted;
a RED test asserting the hook runs on a goal-continuation-shaped prompt would cover it.