chore: standardize on .NET 10 (LTS) across docs, codegen, CI, and fixtures

[MarkMichaelis]

Summary

Standardizes the repository on .NET 10 (LTS), resolving the prior inconsistency where the documented baseline said .NET 9+, the cloud runtime used 10.x, CI used 8.0.x, codegen emitted net8.0, and fixtures used net9.0.

Why .NET 10

• .NET 10 is the current LTS, so the "stay on LTS" rationale for net8.0 now points to 10.
• The Copilot cloud runtime already provisions 10.x (copilot-setup-steps.yml).
• net9.0 is STS and effectively end-of-life.

Changes

• Baseline docs .NET 9+ -> .NET 10+ (CLAUDE.md, copilot-instructions.md); project.instructions.md.template example -> .NET 10
• api-wrapper-scaffold codegen + templates net8.0 -> net10.0 (generate-wrapper.js, Tests.csproj.tmpl, secret-gate ci.yml.tmpl SDK 8.0.x -> 10.0.x)
• validate-instructions.yml SDK 8.0.x -> 10.0.x
• run.Tests.ps1 fixtures net9.0 -> net10.0
• Matching test assertions updated (codegen-pipeline, csharp-templates, oauth-templates, secret-gate)

Intentionally left unchanged

• CLAUDE.project.md.template -- illustrative consumer example of a pinned older runtime
• docs/dogfood/tripit-dry-run-report.md -- point-in-time historical report
• sln-emit.js comments -- accurate history of when .slnx support arrived
• csharp.instructions.md ".NET 7+" -- accurate feature-availability note

Verification

Full .github Pester suite (CI parity): 366 passed, 0 failed, 4 skipped (env-only skips). The agent-e2e, csharp-templates, and oauth-templates tests build the emitted net10.0 projects successfully on the local .NET 10 SDK.

Closes #188