Bound CLI migration readiness by process lifecycle

[Chris0Jeky]

Summary\n\n- Race the migration startup-trace observer against the real CLI process task and one absolute default process deadline.\n- Fail promptly on early process completion with exit code, bounded redacted stderr, and the last trace snapshot.\n- Keep the existing cancellation/reap path and production process deadline unchanged.\n- Add deterministic regressions for early completion and missing readiness.\n\n## Verification\n\n- dotnet test backend/tests/Taskdeck.Cli.Tests/Taskdeck.Cli.Tests.csproj -c Release -m:1 --filter FullyQualifiedName~CliTestHarnessTests — 24/24\n- dotnet test backend/tests/Taskdeck.Cli.Tests/Taskdeck.Cli.Tests.csproj -c Release -m:1 — 208/208\n- git diff --check origin/main...HEAD — passed\n\n## Docs\n\nNo canonical docs changed; this is a bounded CLI test-harness repair.\n\n## Scope\n\nRefs #1665

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