Fix VSIX packaging and add publish tooling for release

[helly25]

Summary

Makes the extension packageable and sets up automated publishing. vsce package was failing before this change, so no release could be built.

What was wrong

• Packaging crashed. The incomplete .vscodeignore let the .trunk/ directory into vsce's secret scanner, which died with EISDIR: illegal operation on a directory — no .vsix was ever produced.
• Dev files would ship. .github/, out/test/, eslint.config.js, package-lock.json, and .DS_Store were all being included.
• No publish tooling. @vscode/vsce wasn't a dependency and there were no package/publish scripts.
• CI push trigger pointed at master while the default branch is main, so push CI never ran.

Changes

• Expanded .vscodeignore to exclude .trunk/**, .github/**, out/test/**, eslint.config.js, package-lock.json, .pre-commit-config.yaml, and **/.DS_Store.
• Added @vscode/vsce@^3.9.1 devDependency and package / publish npm scripts.
• Fixed the CI push trigger to watch main.
• Added .github/workflows/release.yml: on a published GitHub Release it packages the VSIX, runs vsce publish (using a VSCE_PAT repo secret), and attaches the VSIX to the release.

Verification

npm run package now succeeds. Resulting VSIX (9 files, 1.15 MB):

extension/
├─ LICENSE.txt
├─ changelog.md
├─ package.json
├─ readme.md
├─ images/ (iwyu.png, logo.png)
└─ out/extension.js

Before the first automated release

• Create a repo secret VSCE_PAT = Azure DevOps PAT for the helly25 publisher, scope Marketplace > Manage.
• Ensure the GitHub Release tag matches the version in package.json.

Follow-up (not in this PR)

• images/logo.png is 1362×1362 / 1.1 MB; Marketplace renders icons at 128×128. Optimizing it would shrink the VSIX considerably. Left as-is to avoid touching the branding asset without sign-off.
• CI runs only pre-commit, not npm run compile / npm test. Worth adding.

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