feat(common): export SDK version

[nanookclaw]

Adds a public sdkVersion export from @temporalio/common, backed by the package version so consumers do not need to import package.json or internal package metadata directly.

The new focused AVA test imports the public export and independently reads packages/common/package.json, so it verifies the runtime wiring and version value rather than comparing the export to itself.

Closes #417.

Verification

• pnpm install --frozen-lockfile
• git submodule update --init --recursive packages/core-bridge/sdk-core
• pnpm run build:protos
• pnpm -C packages/common run build
• pnpm -C packages/common run test
• pnpm exec prettier --check packages/common/src/index.ts packages/common/src/pkg.ts packages/common/src/__tests__/test-sdk-version.ts

[CLAassistant]

All committers have signed the CLA.

[nanookclaw]

Rebased the branch on current main and pushed 095e421d.

Verification after the rebase:

• pnpm install --frozen-lockfile
• git submodule update --init --recursive packages/core-bridge/sdk-core
• pnpm run build:protos
• pnpm --filter @temporalio/common run build
• pnpm --filter @temporalio/common run test (22 passed)
• pnpm exec prettier --check packages/common/src/index.ts packages/common/src/pkg.ts packages/common/src/__tests__/test-sdk-version.ts
• git diff --check