feat: whitebox/greybox/repl accept a GitHub URL (auto-clone)

`whitebox <arg>`, `greybox --repo <arg>`, `tui --repo`, and the REPL `/repo`
now accept a git URL (https://github.com/owner/repo[.git], git@…, ssh://, *.git)
or an `owner/repo` shorthand. A new resolve_source() shallow-clones it into
<base>/repos/<name> (cached, .gitignored) and reviews it; existing local paths
are used unchanged. Works identically with API-key (--model) and --subscription.

Verified: `neurosploit whitebox https://github.com/digininja/DVWA --offline`
clones DVWA and runs the 78 code agents over 120KB of source.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -104,3 +104,7 @@ data/repl_runs.json
 data/repl_history.txt
 .neurosploit/
 /tmp/*
+
+# Cloned source repos (whitebox/greybox from a git URL)
+repos/
+neurosploit-rs/repos/