vscode ext: esbuild-bundling follow-ups from #241 review

[Hessesian]

Follow-ups from review of #241 (bundle vscode-languageclient with esbuild). The fix itself is correct and merged — these are non-blocking DX/robustness items surfaced during review (mine + Copilot's).

• Local dev/debug is broken until a manual build step runs. main now points at contrib/vscode/dist/extension.js, which is only produced by the vscode:prepublish esbuild hook (run automatically by vsce package, but not by npm install). Anyone launching the extension via F5 / --extensionDevelopmentPath against a fresh checkout will hit a missing-module error. Add a predebug/watch script or a CONTRIBUTING note documenting npm run vscode:prepublish as a required step before local debugging. (Copilot)
• npx esbuild can silently fetch an unpinned version. If contrib/vscode/node_modules isn't populated when the prepublish hook runs, npx esbuild falls back to downloading a version from the registry instead of using the pinned ^0.25.0 devDependency. Low practical risk today (bundling would fail earlier anyway since vscode-languageclient wouldn't resolve without node_modules), but safer to invoke via node_modules/.bin/esbuild or npm exec --no so it never silently drifts. (Copilot)
• Anchor the .vscodeignore exclusion. The added extension.js line is unanchored, so by gitignore semantics it should also match dist/extension.js — empirically it doesn't (vsce appears to special-case the file referenced by main), but relying on that undocumented behavior is fragile. Change to /extension.js to make the intent (exclude only the top-level raw source) explicit and future-proof against any change in vsce's matching behavior.
• Add dist/ to contrib/vscode/.gitignore. It currently only ignores node_modules/. Since the prepublish hook writes dist/extension.js into the working tree, a contributor who builds locally and runs git add . could accidentally commit the generated bundle.

Reference: PR #241 (#241), merged as 0aa292c.