test: golden-file snapshot tests for milestone + tagger

[skjnldsv]

Adds snapshot-style coverage for the milestone mover and tagger in tools/release/, mirroring the "scenario -> expected" feel of the old bash harness, in PHP.

What

• tests/Support/MatchesSnapshots - golden-file helper. A test renders a string and compares it to tests/snapshots/<name>.snap. Re-run with UPDATE_SNAPSHOTS=1 to regenerate after an intended change.

• tests/Support/Journal - renders the recorded GitHub mutations as plain English, so the .snap files read like a description of what a release does, e.g.:

  move issue #100 to milestone #11 in nextcloud/server
  close milestone #10 in nextcloud/server
  create milestone "Nextcloud 33.0.6" in nextcloud/server
  

  nextcloud/activity: OK on stable34 (created v34.0.1)
  nextcloud/server: SKIPPED on stable34 (already tagged)
  

• MilestoneSnapshotTest - patch, first stable, first beta, missing-next, due dates, multi-repo, pre-release no-op.

• TaggerSnapshotTest - mixed run (create / skip / default-branch fallback / server-immutable / no-branch fail), force, dry run.

Complements the focused assertions in MilestoneUpdaterTest / RepoTaggerTest: the snapshots pin the whole sequence as a readable artifact.

The updater bash snapshot harness (tests/updater-script/) is untouched. 88 tests green; snapshots deterministic.