#4026 swamp hangs silently when an extension source directory contains the swamp repo itself

Observed with swamp 20261026.145245.0. Setup: a swamp repo at ext/example (swamp repo init), then from inside it: swamp extension source add .. (the extension root, which contains example/). After that, swamp model type search clef --json hangs indefinitely with no output, even at --log-level debug, and ignores SIGTERM from timeout (had to use timeout -s KILL). The same repo works in about one second once the source points at ../extensions/models instead, and an equivalent repo outside the extension directory works with the extension root as its source. Expected: the source scan skips nested swamp repos (.swamp/, .swamp.yaml) or at least fails with an error naming the recursive source, rather than hanging. Found while building an example harness for the cloudflare-clef extension (lab 4023).