refactor: use InstanceState context in File service

[kitlangton]

Summary

• stop packages/opencode/src/file/index.ts from reading ambient Instance.directory, Instance.worktree, and Instance.project
• read the current file service context from InstanceState.context in scan, status, read, and list
• keep file path boundary checks local to the service so the migration does not depend on ALS-backed sync getters

Testing

• bun typecheck