#4027 swamp extension source add rejects a relative direct-content path that works as an absolute path

Observed with swamp 20261026.145245.0. From a swamp repo at ext/example, swamp extension source add ../extensions/models fails with: No extensions found at '../extensions/models'. Expected either 'extensions//' subdirectories ... OR files declaring extension exports ... directly in the path. Running the same command with the absolute path of that directory succeeds and the model type is then discoverable. A relative standard-layout path (swamp extension source add ..) is accepted, so relative resolution appears to differ between the standard and direct-content layouts. Expected: relative direct-content paths resolve against the current directory the same way absolute paths do. Found while building an example harness for the cloudflare-clef extension (lab 4023).