fix(core): fail closed on git transport errors - #255

Summary

• preserve normal Git non-zero exit semantics
• fail closed when the Git process cannot start or transport fails
• cover both repository discovery and root-commit lookup through ProjectV2.resolve

Verification

• packages/core project + git tests: 16 passed
• packages/core typecheck passed
• mutation restoring the old catch makes both regression paths fail

Closes #239