[codex] Fix workspace manifest and use validation

[Widthdom]

Summary

• Bound workspace manifest member path strings and default_db_name, while keeping the existing member-count cap.
• Reject unknown workspace index_strategy values instead of silently falling back to per-member behavior.
• Make workspace use reject missing members and ambiguous same-basename members before writing active workspace state.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --filter "FullyQualifiedName~WorkspaceCommandRunnerTests" -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet build CodeIndex.sln -p:UseSharedCompilation=false after each issue commit
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Codex adversarial review: No blocking/actionable issues found.

Documentation / Changelog

• Updated DEVELOPER_GUIDE.md workspace manifest and workspace use contracts.
• Added changelog fragments:
  • changelog.d/unreleased/3216.fixed.md
  • changelog.d/unreleased/3041.fixed.md
  • changelog.d/unreleased/3217.fixed.md
  • changelog.d/unreleased/3165.fixed.md

Follow-up candidates

None.

Fixes #3216
Fixes #3217
Fixes #3165
Fixes #3041