Delete the store package

[SaladDay]

Deletes the store package and guards the layering. This finishes issue #1.

• Tests: services/core/internal/store had no production code left. Its 243 integration test files move with git mv to services/core/tests/integration (package integration). The changes stay mechanical: the export_test.go forwarders are gone, and three files are renamed. The set of top-level test names is the same before and after (411), and the 3 shards still split them exactly once.
• Adapter move: runtimehistory/postgresreader moves to persistence/postgres/runtimehistorypg, beside the other adapters.
• Layering guard: cmd/server/layering_test.go uses only the standard library and runs in backend (core). Outside internal/persistence and internal/db, it rejects any non-test file that imports a persistence package, the generated queries or pgx. The rule is stated once in services/core/IMPLEMENTATION.md.
• CI and docs (paths and names only):
  • The Makefile check-core-store target becomes check-core-integration, with CORE_INTEGRATION_PACKAGE and OAC_CORE_INTEGRATION_SHARD.
  • The check.yml matrix part is now integration, still in 3 shards. The required check is still check.
  • The api-acceptance.yml path changes, and two test names that no longer exist are dropped from its -run pattern.
  • ci_plan_test.py and ci_shard_test.py samples are updated.
  • Docs (en and zh): development.md, maintainers.md, harness-onboarding.md, model-execution.md, plus services/core/README.md.
• Dead code removed: verifyEnvironmentEventsWithSDK and tests/official_environment_events.py.

Blind review found no blockers or majors. The minors and nits are fixed. Folding fixtureDB into the integration Store fixture is left for a follow-up.

Checks:

• go build ./..., go vet ./services/core/..., and go test ./services/core/cmd/server (including TestLayering).
• api and execution tests.
• Integration shards 1–3 with the database (shard 1 rerun after the rebase), with no failures.
• check-names, ci_plan_test, ci_shard_test, and the website translations test.
• make openapi produces no diff.

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