test: fix proxy e2e approval mode; add auto-approve e2e coverage

JettyProxyFixture previously wired AutoApprovalGateway into
PushFinalizerFilter, which caused the two ProxyModeE2ETest block-then-
approve tests to fail (clean pushes were forwarded immediately instead
of being held for review).

Changes:
- JettyProxyFixture: add overloaded constructor accepting a
  Function<PushStore, ApprovalGateway> factory for the proxy path;
  default to UiApprovalGateway (block-then-approve, production default)
- ProxyModeAutoApproveE2ETest: new e2e class that passes
  AutoApprovalGateway::new to the fixture; covers clean pushes
  succeeding immediately and validation rejections still applying

docs: migrate plan docs to GitHub issues (#8-#15)

- plan-config-refactor      -> #8
- plan-db-layer-refactor    -> #9
- plan-mirror-sync          -> #10
- plan-internal-pr-gate     -> #11
- plan-registry-abstraction -> #12
- plan-server-dashboard-decoupling -> #14
- plan-user-management      -> #15

Author: coopernetes