MCP Track B Phase B3: expose initial read-only personal SimpleChat MCP tools

## Summary

Implement the initial read-only personal-scope inbound SimpleChat MCP tools from #1013 after the inbound auth/governance foundation is in place.

## Initial Tool Set

- `show_user_profile`
- `list_conversations`
- `get_conversation_messages`
- `list_personal_documents`
- `list_personal_prompts`
- `search_personal_documents`
- `list_agent_template_tags`

## Scope

- Reuse existing SimpleChat authorization helpers and service functions where possible.
- Return minimized profile data; do not return raw token claims by default.
- Ensure conversation and message access is limited to the delegated user or valid collaboration access.
- Ensure personal documents/prompts are limited to the delegated user's personal workspace.
- Restrict search to personal documents only.
- Require pagination and maximum result limits.
- Respect the agent template gallery feature flag for `list_agent_template_tags`.
- Add audit logging per tool call without logging secrets or sensitive document/message content.

## Acceptance Criteria

- [ ] Each initial tool is explicit in the inbound MCP registry.
- [ ] Each tool checks inbound MCP auth and governance before execution.
- [ ] `show_user_profile` returns minimized profile data.
- [ ] `list_conversations` returns only visible conversations for the delegated user.
- [ ] `get_conversation_messages` rejects inaccessible conversations.
- [ ] `list_personal_documents` returns only the delegated user's personal documents.
- [ ] `list_personal_prompts` returns only the delegated user's personal prompts.
- [ ] `search_personal_documents` enforces personal scope and bounded `top_n`.
- [ ] `list_agent_template_tags` respects the agent template gallery feature flag.
- [ ] Tests cover positive and negative access paths for every tool.

## Notes

Parent: #1013
Depends on the inbound auth/governance foundation issue.
Planning doc: `docs/explanation/features/MCP_PLUGIN_ROBUSTNESS_PLAN.md`
Priority: P1
Size: L