ci: support manual nightly CBWeb publishing

[peternhale]

Summary

• retain universal and web VSIX assets on nightly GitHub prereleases
• add internal-only manual publishing through registries: none and publish-web-vsix
• validate the pnpm release lifecycle on the test branch

Validation

• pnpm run test:distribution-policy
• pnpm lint: existing no-console warnings only
• pnpm test: 124 suites / 1547 tests passed
• live nightly release: https://github.com/forcedotcom/debug-log-analyzer/actions/runs/31712275095

Dependency

Requires salesforcecli/github-workflows#170 to merge first, then workflow references can move from the test branch to main.

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @actions-user to sign the Salesforce Inc. Contributor License Agreement.