Auto-merge every Dependabot tier (drop semver-major skip)

[ptr727]

Summary

The merge-bot still skips semver-major Dependabot bumps (holding them for human review) via the version-update:semver-major guard on the merge step. Per a maintainer decision, Dependabot should auto-merge every tier including semver-major — the required CI checks are the gate, not the version magnitude.

Current (drifted) behavior

.github/workflows/merge-bot-pull-request.yml merge-dependabot job carries:

# Skip semver-major NuGet bumps so they land via human review; ...
if: ... (steps.metadata.outputs.update-type != 'version-update:semver-major')

Desired behavior (already implemented in Utilities and LanguageTags)

Every tier auto-merges, semver-major included: the required checks are the gate, not the version bump.

Requested change

• Remove the version-update:semver-major condition from the Dependabot merge step.
• Remove the now-unused Get dependabot metadata step (it exists only to feed that guard) if nothing else consumes it.
• Update the repo's workflow contract docs (WORKFLOW.md D8.2 / diagram / trace, AGENTS.md invariant) accordingly.

Reference implementation: ptr727/homeassistant-purpleair (merge-bot + WORKFLOW.md/AGENTS.md updated to the every-tier policy). Root cause is template drift — see the companion issue in ptr727/ProjectTemplate.

Please also verify no already-open major Dependabot PRs are stuck (auto-merge only fires on opened/reopened, so existing ones may need a reopen after the fix lands).