Prepare Open VSX submission workflow

[chaiwithcode]

Summary

• Add a manual Open VSX workflow for the first submission of an existing GitHub release VSIX.
• Verify the release tag and packaged extension identity before upload; create the chaiwithcode namespace if needed.
• Pin the Open VSX CLI in both the manual workflow and future tag releases.
• Document the required publisher agreement, token secret, namespace verification, and first-publication steps.

Verification

• Confirmed chaiwithcode.xbridge and the chaiwithcode namespace are not yet listed in Open VSX.
• Verified the v1.4.2 release VSIX SHA-256 against the GitHub release asset and inspected its publisher, name, version, license, and icon.
• Built a current local VSIX successfully (20 files, 1.18 MB).
• npm run lint and npm test passed (19 tests).
• Workflow YAML, shell syntax, and git diff --check passed.

No Open VSX publication is triggered by this PR. The owner must sign the Open VSX Publisher Agreement and configure OVSX_PAT before running the manual workflow.