Pin remote commands to the synced environment workspace

[ethgr0wth]

Production regression

After PR #11, a live remote terminal listed the Runtime B service source tree instead of the selected Keystone environment. The deployed runtime was executing from its process cwd.

Fix

• Capture the absolute workspace returned by sync_workspace.
• Require that path to end in workspaces/<session-id>.
• Use it as the explicit root for every terminal and agent shell command.
• Send the environment ID with every run_code request.
• Refuse execution when sync omits the workspace or the returned root is invalid.
• Keep relative cd state constrained beneath the pinned workspace.

This is fail-closed and protects Keystone even while an older Runtime B is deployed.

Validation

• npm run build
• git diff --check
• Regression test launched execution from a fake Runtime B source directory and confirmed only environment files were listed.
• Negative test confirmed the service source directory is rejected as an invalid workspace root.