feat(plugin-vscode): short-circuit dry-run builds, emit vscode-package-plan.json

[hikaruhuimin]

Closes #156

/claim #156

Summary

• short-circuit plugin-vscode dry-run builds before any vsce/npm exec checks
• write a deterministic vscode-package-plan.json artifact with package dir, target, expected VSIX, and command
• keep real builds on the existing npx --yes vsce package path
• add tests proving dry-runs skip exec and real builds still invoke vsce packaging

Verification

• pnpm exec vitest run packages/targets/plugin-vscode/src/index.test.ts
• pnpm --filter @profullstack/sh1pt-target-plugin-vscode typecheck
• pnpm --filter @profullstack/sh1pt typecheck
• git diff --check