chore(workflows): migrate git-sync SSH key from GitHub secret to Secrets Manager via OIDC

[siddsriv]

Issue

Internal JS-6698

Description

Migrates the git-sync workflow from using a GitHub repo secret (GIT_SYNC_SSH_PRIVATE_KEY) to fetching the SSH key at runtime from Secrets Manager via OIDC role assumption.

Checklist

• If the PR is a feature, add integration tests (*.integ.spec.ts) or E2E tests.
  • It's not a feature.
• My E2E tests are resilient to concurrent i/o.
  • I didn't write any E2E tests.
• I added access level annotations e.g. @public, @internal tags and enabled doc generation on the package. Remember that access level annotations go below the description, not above.
  • I didn't add any public functions.
• Streams - how do they work?? My WebStream readers/locks are properly lifecycled. Node.js stream backpressure is handled. Error handling.
  • No streams here.

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