fix(chat): stop active runs and cancel tool invocations

[AlphaCatMeow]

Summary

• release chat UI state immediately when a user stops an active or queued turn
• propagate cancellation through MCP, shell, managed process, and SSH tool invocations
• add runtime cancellation commands and token-safe cleanup in the Tauri backend
• prevent locally cancelled gateway runs and queued turns from restarting or overwriting stopped state
• keep desktop and gateway web chat/sidebar behavior in sync

Validation

• pnpm --dir crates/agent-gui build
• pnpm --dir crates/agent-gateway/web build
• cargo check --manifest-path crates/agent-gui/src-tauri/Cargo.toml --tests
• 72 targeted Node tests covering stop timing, gateway listeners, memory extraction, abortable invocation, MCP, shell, SSH, and sidebar state
• git diff --check upstream/main..HEAD

Notes

The full frontend test commands exceeded the local runner's fixed 30-second execution window; the focused cancellation suites above completed successfully with no failures.