day10: fix jtw-tools repo URL, git+ prefix, and output path extraction
Three fixes for day10's JTW integration:
1. Default --jtw-tools-repo changed from bogus github URL to correct
tangled.org URL, default branch from "enhancements" to "main"
2. opam pin commands now use git+ prefix (git+https://...) so opam
treats the URL as a git repo instead of trying to download it as
an archive
3. Container output path fixed: jtw opam --path <pkg> writes to
<output>/<pkg>/lib/ but linux.ml was looking at <output>/lib/.
Now correctly reads from <output>/<pkg>/lib/.
Tested end-to-end with seq.0.3.1: jtw-tools layer builds, per-package
artifacts generated, batch assembly produces correct findlib_index.json
with meta_files key and content-hashed paths.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>