CI: also publish to Open VSX Registry (Cursor/VSCodium/Windsurf)

[jack-champagne]

Follow-up to the VS Code Marketplace publish workflow.

Once Marketplace publishing lands, extend the release workflow to also publish to the Open VSX Registry so the extension is installable in Cursor, VSCodium, Windsurf, and other non-Microsoft VS Code distros.

Prereqs (not yet done):

• Sign the Eclipse Foundation Open VSX publisher agreement.
• Claim the harmoniqs namespace on open-vsx.org.
• Mint an Open VSX access token (open-vsx.org → user settings → Access Tokens).
• Add OVSX_TOKEN as an Actions secret on this repo.

Workflow change:

• Add an ovsx publish step mirroring the vsce publish step (same per-target vsixes + universal fallback), gated on the same non-alpha tag rule.

Deliberately deferred: no OVSX_TOKEN exists yet. Marketplace-only for the first cut.