fix(core): stop generating test-manifest-stub.ts into src/manifest during build

[willgriffin]

Summary

smrtPlugin still has legacy build behavior that writes a generated test-manifest-stub.ts into src/manifest whenever that directory exists. This dirties working trees in consumer packages and mixes generated build artifacts into tracked source.

Current behavior

In packages/core/src/vite-plugin/index.ts, closeBundle() does this:

• writes dist/manifest.json
• checks whether src/manifest/ exists
• if it does, writes src/manifest/test-manifest-stub.ts

This means a package can end up with a modified tracked source file just from running a build.

Why this is a problem

• Generated build/test artifacts should not be written into src/
• It creates noisy dirty worktrees
• It is easy for packages to accidentally keep legacy src/manifest directories around
• The behavior is surprising because SMRT already prefers .smrt/manifest.json and dist/manifest.json in newer flows

Reproduction

1. In a package using smrtPlugin, create src/manifest/
2. Run vite build
3. Observe src/manifest/test-manifest-stub.ts being generated by the plugin
4. If that file is tracked, the checkout is now dirty

Evidence

Current logic in packages/core/src/vite-plugin/index.ts:

• const manifestDir = resolve(projectRoot, "src/manifest")
• if (existsSync(manifestDir)) { ... writeFileSync(resolve(manifestDir, "test-manifest-stub.ts"), stub) }

Expected behavior

Build output should stay in build/test output locations, not in src/.

Proposed fix

One of:

1. Remove this legacy src/manifest/test-manifest-stub.ts generation from closeBundle() entirely
2. Generate the stub into .smrt/ or dist/ instead
3. Make the stub output path explicit/configurable and default it away from src/

I would lean toward removing the implicit src/manifest write entirely unless there is still a hard compatibility requirement.

Context

This came up in anytown.ai while cleaning up a package that kept getting dirtied by a generated src/manifest/test-manifest-stub.ts on local builds. Removing the stale src/manifest/ directory fixes that package locally, but the root cause is still in smrt-core.