Track remaining inbound MCP tool implementations

## Summary
Track the remaining inbound SimpleChat MCP tool implementations after the initial governed personal read tools shipped through `0.250.083`.

## User Value
Approved MCP clients should be able to use more SimpleChat capabilities without weakening existing delegated-user, workspace, governance, audit, and data-protection boundaries.

## Proposed Behavior
Continue implementing inbound MCP tools in small, governed slices. The next planned tool is `search_personal_documents`. `execute_workflow` should remain later because it is execution/write-like and needs stronger guardrails. `send_personal_chat_message` is explicitly out of scope for now.

## Scope
- Implement `search_personal_documents` for delegated personal scope only.
- Return bounded, metadata-safe search results using existing SimpleChat document/search authorization boundaries.
- Keep `execute_workflow` planned but gated behind separate workflow ownership, execution governance, rate limits, audit, distributed run-lock, token/cost logging, and bounded response metadata.
- Keep group, public, all-scope, admin, and app-only tools disabled until explicit workspace-role/governance tests are designed.
- Do not implement `send_personal_chat_message` as part of this tracking issue.

## Acceptance Criteria
- [ ] `search_personal_documents` is implemented with delegated-user-only scope, bounded `top_n`, and no caller-supplied `user_id` or arbitrary scope override.
- [ ] Search results are metadata-safe and do not return unbounded document content.
- [ ] Existing SimpleChat personal document access/search boundaries are reused or explicitly preserved.
- [ ] Tool registry, docs, release notes, functional tests, and live MCP smoke-test evidence are updated for each shipped tool slice.
- [ ] `execute_workflow` has a separate design/implementation gate before it is enabled.
- [ ] `send_personal_chat_message` remains skipped until a later explicit decision.

## Notes
- Follows the MCP robustness plan Track B Phase B3.
- Builds on `0.250.083`, where `list_conversations`, `get_conversation_messages`, `list_personal_documents`, `list_personal_prompts`, and `list_personal_tags` are live and validated.
- Related prior tracking: microsoft/simplechat#1017.