Fix extension diagnostics and trust status reporting

[Widthdom]

Summary

• Normalize extension discovery and load diagnostics into stable sanitized categories/messages.
• Share isolated worker process launch setup between symbol extraction and hook callback workers.
• Surface accepted extension trust overrides in CLI/MCP status output and document the status contract.

Validation

• dotnet build CodeIndex.sln -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• git diff --check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Targeted tests for Standardize plugin and hook discovery exception taxonomy #3698, Normalize plugin and hook assembly-load, dependency-resolution, and activation diagnostics #3701, Consolidate isolated worker process-launch contracts #3703, and Surface hook and plugin trust environment overrides in status diagnostics #3735 on net8/net9.
• Full tests/CodeIndex.Tests run initially found only the updated Normalize plugin and hook assembly-load, dependency-resolution, and activation diagnostics #3701 hook diagnostic expectation; fixed in follow-up commit and reran the affected tests on net8/net9.
• Codex adversarial review completed; follow-up focused review reported: No blocking/actionable issues found.

Issues

Fixes #3698
Fixes #3701
Fixes #3703
Fixes #3735