v0.138.0: MCP management upgrades, Mermaid export, and stability fixes

New features

- Interactive MCP management - Add, remove, and list MCP servers interactively
- MCP credential variables - MCP server configs can now expand credential variables at runtime
- Configurable MCP call timeout - Set a custom timeout for MCP tool calls
- mcp off shortcut - Quickly disable MCP servers with a new shortcut
- MCP servers for custom droids - Choose which MCP servers are available to custom droids
- Per-server MCP risk - Configure command risk levels for each MCP server
- Mermaid export - Export Mermaid diagrams from chat (app)

Improvements

- Highlighted new models - New models now appear highlighted at the top of /models
- MCP embedded resources - MCP servers' embedded resources are now surfaced in chat
- Composer loading state - The message composer now shows a loading state (app)
- Session archiving - Unified the session archive flow across the app and desktop (app)
- Pinned messages - Clicking a pinned message now scrolls to its source (app)

Bug fixes

- Turn cancellation - Canceling a turn now takes effect immediately
- Session rename - Renamed session titles are now trimmed of extra whitespace (app)