CIMNotebook: Frontend UI - Execution, Outputs & Cancellation

[spah-soptim]

Objective: Users can run and cancel cells from the frontend and view formatted results.

Description:
Wire up the backend logic from Sub-Issue 3 to the VS Code Notebook UI.

• Implement the NotebookController (sequential execution).
• Parse # [endpoint=...] directives via regex to determine the execution target.
• Call sendRequest to the LSP server.
• Output Rendering: Render SELECT/ASK as a markdown table (text/markdown) and JSON. Render CONSTRUCT/DESCRIBE as fenced Turtle.
• Cancellation: Pass the CancellationToken to the server. Implement a CancellationWatchdog in the backend that calls QueryExecution.abort() alongside Jena timeouts.

Acceptance Criteria:

• The "Run" button executes the query and correctly renders the table or Turtle.
• Running queries can be stopped via the frontend "Stop" button (watchdog triggers).
• Timeouts (max duration) are handled gracefully.
• Documentation updated to cover execution, directives, and cancellation.