Fix CLI JSON and terminal output controls

[Widthdom]

Summary

• Suppress ANSI styling for JSON-producing CLI modes, including --json-envelope, even when color is forced.
• Enforce UTF-8 no-BOM stdout for real redirected CLI output while preserving test/captured writers.
• Tighten interactive terminal detection for TERM=dumb, CI, missing Unix terminal hints, redirected output, and captured output.
• Fix the macOS/net9.0 CI failure by moving ready-bit updates to provider-managed immediate SQLite transactions (SetReadyBit concurrent writers can start nested SQLite transaction #2676).
• Document the CLI output contract and add issue-specific changelog fragments.

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~ConsoleUiTests"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --filter "FullyQualifiedName~ProgramRunnerTests|FullyQualifiedName~ConsoleUiTests"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -f net9.0 --no-restore --filter "FullyQualifiedName~ConcurrencyTests.SetReadyBit_ConcurrentWriters_DoNotLoseFlags"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -f net9.0 --no-build --filter "FullyQualifiedName~ConcurrencyTests"
• dotnet run --project tools/CodeIndex.Changelog -- check
• Codex adversarial review: first pass found a --json-envelope ANSI-suppression gap; fixed in Handle JSON envelope console output (#1956). Second pass reported No blocking/actionable issues found.
• Codex adversarial review for SetReadyBit concurrent writers can start nested SQLite transaction #2676 CI fix: No blocking/actionable issues found.

Documentation and changelog

• Updated README.md output controls in English and Japanese.
• Updated DEVELOPER_GUIDE.md with CLI JSON encoding and terminal-control implementation rules.
• Added changelog fragments:
  • changelog.d/unreleased/1833.fixed.md
  • changelog.d/unreleased/1834.fixed.md
  • changelog.d/unreleased/1953.fixed.md
  • changelog.d/unreleased/1956.fixed.md
  • changelog.d/unreleased/2676.fixed.md

Issues

Fixes #1956
Fixes #1953
Fixes #1833
Fixes #1834
Fixes #2676

Follow-up candidates

• None.