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

## 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.

Start with createDefaultInstalledAgentSource and the existing coverage in installSourcesFeed.spec.ts and installSourcesInstalledProvider.spec.ts. Trace how the installed source reaches feed materialization, then add one shared test seam and cover feed-driven prune, rollback/commit mirror pruning, and package update re-resolution through the installed-source path.

Written by the indexing model from the issue text.