feat: require saving a workspace before compiling

[kbuffardi]

Summary

Implements #69 by making the fresh Hello World document explicitly untitled and ephemeral until it is saved into a selected workspace folder.

• Removes the phantom Explorer file and source-only session persistence.
• Gates all terminal commands except help until a workspace is open.
• Disables Compile and Compile & Run until the workspace contains a .cpp file, with action-level guards as a backstop.
• Uses folder selection followed by filename entry for untitled Save and Save As, then creates and binds the workspace file.
• Clears terminal, diagnostics, run intent, tabs, workspace state, and cached binary state when starting a project or opening a folder.
• Updates E2E coverage for terminal gating, no-storage sessions, untitled compilation rejection, and workspace-gated compilation.

Verification

• npm run lint
• npm run test:e2e (175 passing)
• npm run build

Build completes with the existing webpack bundle-size warnings.

Closes #69