fix(pr): preserve GitHub lookup failures during branch PR resolution

[unohee]

Summary

• Distinguish a genuine no-open-PR result from GitHub/provider lookup failures.
• Preserve provider failures as typed errors with the original cause and actionable auth/network guidance.
• Add regression coverage for absent PR, provider failure, cause preservation, and resolvePR propagation.

Verification

• Targeted vitest suite: 73 tests passed.
• Typecheck: passed.
• RED baseline: 6 new tests failed against pre-change implementation.