feat(vscode): bundle release binaries into platform VSIX

[jscaltreto]

Parent: #33

Summary

Bundle the downstage binary into platform-specific VSIX packages, but do not add a second cross-compile pipeline in the VS Code workflow. The release workflow should reuse the platform binaries already produced by GoReleaser and package those into target-specific VSIX artifacts.

The VS Code workflow should be reframed as CI only: lint, compile, and packaging smoke-test. Release artifact production belongs in the release workflow.

Targets

• linux-x64
• darwin-x64
• darwin-arm64
• win32-x64

Versioning and changelog ownership are tracked in #47. This issue is about producing platform-specific VSIX artifacts that bundle the release binary and align extension packaging with the existing release pipeline.

Direction

• GoReleaser remains the source of truth for platform binaries
• The release workflow packages VSIX files from local GoReleaser outputs in dist/
• The VS Code workflow remains CI and does not own cross-compilation or release publishing
• The extension should prefer an explicitly configured downstage.server.path, then a bundled binary, then PATH

Work

• Update package.mjs to support release packaging from an already-built binary path
• Update package.mjs to emit platform-specific VSIX packages via vsce package --target <platform>
• Update package.mjs to copy the root CHANGELOG.md into the staged extension during packaging
• Update extension activation to find the bundled binary (fall back to PATH)
• Update the release workflow to package and publish platform VSIX files using GoReleaser outputs
• Keep the VS Code workflow focused on CI: lint, compile, and packaging smoke-test
• Consume the version set by the release flow so packaged VSIX metadata matches the release tag
• Update extension README with the fact that the binary is bundled and that release notes come from the shared root changelog

Non-goals

• Adding a separate cross-compile matrix to the VS Code workflow
• Rebuilding release binaries in Node packaging scripts when GoReleaser already produced them