fix: stabilize manifest paths on Windows

[jackwener]

Summary

• normalize manifest sourceFile paths to POSIX separators before writing cli-manifest.json
• centralize manifest serialization and keep cli-manifest.json newline-stable
• use a Windows junction for directory symlink plugin discovery tests, matching production symlink behavior
• add regression coverage for manifest path normalization and serialization

Supersedes the still-draft PR #1052 with the same scoped fix on current main.

Verification

• npx vitest run src/build-manifest.test.ts src/engine.test.ts --project unit
• npm run typecheck
• npm run build
• git diff --check
• npx vitest run src/build-manifest.test.ts src/engine.test.ts src/cli.test.ts src/diagnostic.test.ts --project unit
• OPENCLI_DAEMON_PORT=29125 npm test