fix: map missing git refs separately

[jlitola]

Summary

• classify backend REF_NOT_FOUND responses as a dedicated non-retryable code
• show repo/ref-specific CLI guidance instead of path narrowing or code files hints
• sync bun.lock for the existing core-internal zod dependency

Tests

• bun test src/shared/code-navigation-error-map.test.ts
• bun test src/commands/code/read.test.ts
• bun run build
• bun test