Add outline JSON projection and paging controls

[Widthdom]

Summary

• Add cdidx outline --json controls for field projection, symbol-kind filtering, and pagination via --outline-fields, --kind, --limit / --top, and --cursor outline:<offset>.
• Emit paging metadata for controlled outline JSON responses, including total_symbol_count, returned_symbol_count, cursor_offset, next_cursor, and has_more.
• Update CLI help/schema, user/developer docs, tests, and changelog fragment.

Validation

• dotnet build CodeIndex.sln --no-restore
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --no-build --filter Issue3986
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --no-build --filter RunOutline
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll outline src/CodeIndex/Cli/QueryCommandRunner.cs --json --kind function --limit 2 --outline-fields name,line,kind --pretty
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll outline src/CodeIndex/Cli/QueryCommandRunner.cs --json --kind function --limit 2 --cursor outline:2 --outline-fields name,line,kind --pretty
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Adversarial review: manual origin/main..HEAD review found no blocking/actionable issues. codex exec review was attempted twice but did not complete because the subprocess shell/model-manager environment repeatedly failed before returning review findings.

Documentation / Changelog

• Updated USER_GUIDE.md and DEVELOPER_GUIDE.md.
• Added changelog.d/unreleased/3986.added.md.

Fixes #3986

Follow-up Candidates

• None.