refactor: centralize build path resolution

[jackwener]

Summary

• add a shared for package root, built entry, manifest, and fetch-adapters path resolution
• switch , , and to use the shared helpers instead of duplicating layout heuristics
• update docs/comments that still referenced the stale path

Validation

• npm test -- --run src/cli.test.ts src/execution.test.ts
• npm run build

Notes

• diagnostic/self-repair safety boundary work is intentionally not included here; that is covered separately in PR fix: add safety boundaries to diagnostic output #806