Keep plugin-vscode dry-run builds side-effect free

[RasputinKaiser]

The plugin-vscode target currently verifies and packages through npx vsce during build, and falls back to a global npm install -g @vscode/vsce if the availability check fails.\n\nThat is surprising for validation/dry-run contexts because the shared harness sets dryRun: true specifically so adapters can avoid network/exec side effects. For this target, a dry-run build can still fetch a CLI or try a global install before users are ready to package.\n\nProposed fix:\n\n- when ctx.dryRun is set, write a deterministic package plan artifact instead of invoking vsce\n- keep real builds on the existing npx --yes vsce package path\n- add tests proving dry-run builds do not call exec and real builds still do\n\nThis keeps target validation safe while preserving the real packaging behavior.

[Robertg761]

Implemented in PR #183: #183

Summary:

• plugin-vscode dry-run builds now short-circuit before any vsce/npm exec checks
• dry-runs write vscode-package-plan.json with package dir, target, expected VSIX path, and command
• real builds remain on the existing npx --yes vsce package path
• tests prove 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