Add feed materialize harness for installed-source update/prune coverage

[curtisman]

Summary

Add a hermetic feed-materialize test harness for the installed-agent source so update/prune behavior can be exercised through the real source path instead of path-record stand-ins.

Background

During the install-source/update-coordination work, several coverage gaps were intentionally deferred because createDefaultInstalledAgentSource does not currently expose a feed dependency injection seam. Direct feed materialization is covered in installSourcesFeed.spec.ts, and update/prune behavior is covered through path-record stand-ins in installSourcesInstalledProvider.spec.ts, but the combined end-to-end feed-through-installed-source cases are not covered.

Coverage To Add

• feed-driven prune-on-swap and prune-after-uninstall through createDefaultInstalledAgentSource
• rollback prune of a real distinct v2 install root, plus the commit mirror where v2 is kept and v1 is pruned
• feed @package update NAME RANGE re-resolution through the installed-source update path

Possible Approach

Add a feedDeps / npmInstall injection seam to createDefaultInstalledAgentSource, or expose a narrower test hook that lets the installed-source tests drive a mocked feed materialize. Prefer one seam that covers all three cases rather than separate production-only hooks.

Source

Deferred from PR #2611 review-log cleanup.