Expose status diagnostics and bound hooks

[Widthdom]

Summary

• Expose bounded unknown-extension path samples in status --json and MCP status.
• Report extractor plugin and pattern-config load diagnostics through status.
• Bound post-extraction hook callbacks, drop timed-out mutations, and expose hooks[].callback_budget_ms.

Validation

• dotnet build CodeIndex.sln -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~IndexCommandRunnerTests|FullyQualifiedName~DocumentationStatusContractTests" -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~ExtractorPluginRegistryTests|FullyQualifiedName~Run_StatusJsonIncludesExtractorPluginDiagnostics|FullyQualifiedName~DocumentationStatusContractTests" -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~PostExtractionHookTests|FullyQualifiedName~DocumentationStatusContractTests" -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~PostExtractionHookTests|FullyQualifiedName~ExtractorPluginRegistryTests|FullyQualifiedName~DocumentationStatusContractTests|FullyQualifiedName~Run_VerboseReportsUnknownExtensionCountAndStatusJsonStampsCount|FullyQualifiedName~Run_StatusJsonCapsUnknownExtensionPathSample|FullyQualifiedName~Run_StatusJsonIncludesExtractorPluginDiagnostics"
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Codex adversarial review of origin/main..HEAD: No blocking/actionable issues found.

Documentation And Changelog

• Updated README.md, DEVELOPER_GUIDE.md, and AGENT_GUIDE.md for the status contract additions.
• Added changelog fragments:
  • changelog.d/unreleased/2822.fixed.md
  • changelog.d/unreleased/2865.fixed.md
  • changelog.d/unreleased/2877.fixed.md

Follow-Up Candidates

• None.

Fixes #2822
Fixes #2865
Fixes #2877