dependency: add isolated git-warp v17 upgrade fixture harness #320

Context

Git Mind is preparing to modernize its WARP substrate from @git-stunts/git-warp 14.x to 17.x and @git-stunts/plumbing 2.x to 3.x. Before changing the dependency boundary, we need a frozen upgrade fixture that proves current Git Mind graph state can be read and upgraded in an isolated environment.

Goal

Add a sanitized Git-native fixture and an isolated Docker upgrade harness so the dependency upgrade is tested against a real old Git Mind/WARP graph without relying on the developer's local checkout, Git config, remotes, hooks, reflogs, or mounted repository state.

Acceptance Criteria

• A sanitized fixture captures the current v5/git-warp-14 repository graph state using Git-native refs, not a raw working-directory tarball.
• The fixture includes the relevant refs/warp/gitmind/* refs and metadata describing the source commit, package versions, expected refs, and intended upgrade target.
• The upgrade harness runs in Docker without mounting this repository into the container.
• The harness copies only explicit artifacts into the container: the fixture, a packed git-mind package, and a small runner script/context.
• The container uses a scrubbed environment and does not depend on host Git config, remotes, credentials, or local checkout paths.
• The harness proves that the new code can open/read the frozen graph state and that the fixture artifact itself is not mutated.
• The harness becomes the safety rail for the @git-stunts/git-warp@17 / @git-stunts/plumbing@3 upgrade.

Notes

This follows the repo fixture strategy: archive exact Git object state only because substrate-upgrade compatibility is the thing under test.