Three fixes, all wanted **before** the 4.0.0 release run (merging this
PR to main is itself the release trigger):

1. **4.0.0 changelog entries** now carry the cutover one-liner from
#2671 and link the [v3→v4 migration
guide](https://docs.stagehand.dev/v4/migrations/v3), matching the 3.0.0
entry's pattern.
2. **3.0.1–3.7.1 history restored** — the consolidated root changelog
seeded on v4-spike jumped from 4.0.0 straight to 3.0.0, silently
dropping 16 releases. Spliced back verbatim from
`packages/core/CHANGELOG.md` on the `v3` branch (that history's
maintenance home going forward).
3. **Release workflow unblocked** — the 4.0.0 publish run
[failed](https://github.com/browserbase/stagehand/actions/runs/31361618186/job/93371433942)
with `EBADDEVENGINES`: `devEngines.packageManager` with `onFail:
"download"` makes every npm command in the repo hard-fail (npm can't
download itself into being pnpm), and `changeset publish` preflights
with `npm info` (the publish itself uses `pnpm publish`, so `catalog:`
deps resolve fine — verified in @changesets/cli 2.31's
`getPublishTool`). `onFail: "warn"` keeps the pnpm nudge without
blocking npm tooling; verified `npm info` now succeeds in-repo.

`consolidate-changelogs.ts --check` passes.