refactor(core): type directory initialization errors - #52649

Summary

• Introduce internal tagged errors for missing and access-denied location directories, preserving the directory and original cause.
• Classify NotFound, PermissionDenied, and macOS Unknown/EPERM during local filesystem initialization; unrelated failures remain defects.
• Preserve workspace-only placement behavior.

Scope

First scoped change toward #50951. Instance-level propagation, configuration discovery, HTTP 404/403 mapping, and Desktop recovery UI are intentionally deferred. This does not resolve the issue yet.

Validation

• bun test test/location-filesystem.test.ts from packages/core: 10 passing tests.
• bun run check: passes.
• Regression mutation restoring the old defect behavior fails the missing-directory, EACCES, and EPERM assertions.
• Formatting and git diff --check pass.