refactor(tests): consolidate Milestone Module — 10 files → 4 (#3754)

* refactor(tests): consolidate Milestone Module — 10 files → 4

Closes #3753

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(changeset): correct fragment format for 3753-consolidate-milestone-tests

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(changeset): add required frontmatter to 3753 changeset fragment

Adds `type: Fixed` and `pr: 3753` frontmatter so docs-lint passes.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(tests): bug-2943 execFileSync timeout 5s→15s for Windows starvation

PR #3754's milestone consolidation reshuffles run-tests.cjs chunk
composition. On Windows/Node 22 under --test-concurrency=4, bug-2943
subprocesses now share concurrency slots with bug-2760-codex-install
subtests (8–15s each), starving past the 5000ms timeout. 15s covers
the observed 13.5s worst case with headroom.

Refs #3753

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>