[codex] Add languages indexed and capability filters

[Widthdom]

Summary

• Add cdidx languages --indexed-only to list only languages present in the current database.
• Add cdidx languages --capability graph|symbols|references to filter the language table by structured capability.
• Update CLI flag schema, help usage, user guide docs, tests, and changelog fragment.

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --filter "FullyQualifiedName~RunLanguages|FullyQualifiedName~CliFlagSchema"
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet format CodeIndex.sln --verify-no-changes
• git diff --check origin/main..HEAD
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false failed only on PostExtractionHookTests.CallbackBudgetExceeded_AddsDiagnosticAndSkipsTimedOutMutation in both target frameworks; isolated rerun passed. Tracked separately in PostExtractionHookTests callback budget test flakes under full suite load #3261.

Documentation And Changelog

• Updated USER_GUIDE.md and cdidx languages --help usage.
• Added changelog.d/unreleased/3000.added.md.

Review

• Adversarial review of origin/main..HEAD: No blocking/actionable issues found.
• codex exec review --ephemeral --base origin/main was attempted but did not complete because its internal tool execution loop stalled; a retry with --ignore-user-config terminated at the Codex usage limit. Local workflow review completed instead.

Follow-Up Candidates

• PostExtractionHookTests callback budget test flakes under full suite load #3261

Fixes #3000