[Bug]: confirmation_policy, security_analyzer, and secrets mutations not persisted to meta.json

Bug Description

When set_confirmation_policy, set_security_analyzer, or update_secrets are called via the API, the change is written to ConversationState/base_state.json but StoredConversation/meta.json is never updated. On next startup, event_service.py lines 1090–1102 read all three fields from stored to initialize the conversation, so the changes are silently reverted after eviction or server restart.

Expected Behavior

API mutations to confirmation_policy, security_analyzer, and secrets persist across conversation eviction and server restart.

Actual Behavior

Changes are lost; the conversation reverts to the values set at creation time. This can be verified by running the existing test suite and adding a test that:

1. Creates a conversation
2. Calls the mutation endpoint (e.g. set_confirmation_policy)
3. Simulates eviction by removing the event service from _event_services
4. Reloads the conversation via _get_or_load_event_service
5. Asserts the updated value is still present

pytest tests/agent_server/ -k "confirmation_policy or security_analyzer or secrets"

The relevant code paths are:

• EventService.set_confirmation_policy (line 1668) — only calls self._conversation.set_confirmation_policy(policy); no self.stored update, no save_meta()
• EventService.set_security_analyzer (line 1677) — same pattern
• EventService.update_secrets (line 1658) — only calls self._conversation.update_secrets(secrets); no self.stored update, no save_meta()

On startup, event_service.py reads all three from stored:

# line 1090
secrets=self.stored.secrets,
# line 1101
conversation.set_confirmation_policy(self.stored.confirmation_policy)
# line 1102
conversation.set_security_analyzer(self.stored.security_analyzer)

Compare to EventService.apply_resume_secrets (lines 352–355) which correctly updates self.stored and calls await self.save_meta() after mutating state.

Acceptance Criteria

• set_confirmation_policy updates self.stored.confirmation_policy and calls save_meta() after mutating ConversationState
• set_security_analyzer updates self.stored.security_analyzer and calls save_meta() after mutating ConversationState
• update_secrets updates self.stored.secrets and calls save_meta() after mutating ConversationState
• Tests verify each change survives a simulated eviction/reload

Additional Context

Discovered while reviewing #4617. tags is currently the only field where API mutations correctly write back to stored and persist to meta.json. Discussed in #proj-agent-server.

Start with EventService.update_secrets, set_confirmation_policy, and set_security_analyzer in event_service.py, then compare them with apply_resume_secrets around lines 352–355. Run pytest tests/agent_server/ -k "confirmation_policy or security_analyzer or secrets" and add coverage for eviction and reload. Done means all three mutations remain present after _get_or_load_event_service reloads the conversation.