Add delta feature

[Copilot]

Adds a devcontainer feature for delta, a syntax-highlighting pager for git diffs, building on the implementation from #104.

Changes

• src/delta/devcontainer-feature.json — Feature manifest; version option defaults to "latest", supports pinning to any X.Y.Z semver.
• src/delta/install.sh — Downloads the musl tar.gz from dandavison/delta GitHub releases. Uses envsubst to expand the archive path template since the directory inside the archive includes the release tag (e.g. delta-0.18.2-x86_64-unknown-linux-musl/delta); --strip-components is derived dynamically from the resolved path depth.
• test/delta/test.sh — Verifies delta --version succeeds and outputs delta.
• .github/workflows/test.yaml — Adds delta to both test-autogenerated and test-scenarios matrices.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add delta feature</issue_title>
<issue_description>build on implementation in #104 </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add delta feature #113

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