feat(go-test): aggregate coverage across multiple integration jobs

Makes post-test:go:coverage merge coverage from multiple integration jobs into one total + threshold gate, closing the last gap before multi-job adopters (e.g. artifact-registry/UAR) can drop their post-test:go:coverage patch.

Closes team-tasks#4559 (closed). Follows mft!34 (extensible .test:go:integration base).

What changed (templates/go-test.yml)

• Glob merge — the coverage job now merges every coverage-integration*.txt profile it receives, not just the single coverage-integration.txt. Backward-compatible: the default single-file case is unchanged.
• Referenceable default needs — extracted to a hidden .go-coverage-needs; the coverage job references it via !reference. Consumers with extra integration jobs add them to needs without re-listing the defaults.

How a consumer uses it

Each extra integration job writes a distinct coverage-integration-<name>.txt (as an artifact), then:

post-test:go:coverage:
  needs:
    - !reference [.go-coverage-needs, needs]
    - job: "test:go:integration:redis"
      optional: true

Documented in templates/README.md (single-job and multi-job examples).

Validation

• Component tests: new merge: multiple integration scenario (three profiles → one total); existing scenarios green.
• Canary-validated end-to-end in release-platform-canary against this branch's commit — two extra integration jobs writing distinct profiles + the !reference needs override → coverage job merged all three:

  Found integration coverage: coverage-integration-alpha.txt coverage-integration-beta.txt coverage-integration.txt
  Total coverage: 44.7%

  Pipeline: release-platform-canary #2649098615.

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