fix: surface unresolved repo ref suggestions

[jlitola]

Summary

• parse REF_NOT_FOUND available_refs from code-navigation GraphQL error extensions
• expose repoUrl, requestedRef, and availableRefs in CLI/MCP error details
• append did-you-mean text only when the backend message does not already include suggestions

Verification

• code reviewer: no findings
• bun test packages/core-internal/src/services/code-navigation-service.test.ts packages/mcp/src/shared/code-navigation-error-map.test.ts packages/mcp/src/shared/unified-search-response.test.ts
• bun run build
• bun test
• dev CLI JSON: github:Docs/Tutorial: Introducing a submit button in the comment form react/react#19.2.0 returns REF_NOT_FOUND with details.availableRefs
• dev MCP JSON/text: github:Docs/Tutorial: Introducing a submit button in the comment form react/react#19.2.0 surfaces v19.2.0 suggestion

Notes

• github:update summary to auto openai/codex#1.2.3 currently returns REF_NOT_FOUND without availableRefs because the dev backend does not suggest refs for that case yet; the client now handles future available_refs coverage without another change.