fix: stabilize Windows and Linux CI tests

[DevMando]

The new Windows/Linux test matrix exposed environment assumptions that the local Windows runs did not catch. This PR addresses the failures without skipping the affected tests or disabling parallel execution across the suite.

Root causes and fixes

• CI forces ANSI escape sequences into output. Plain-text layout checks now explicitly disable ANSI in their test console profiles, preserving the actual spacing and order assertions.
• Linux clipboard tests supplied a fake reader but were stopped by a Windows-only shortcut gate. The prompt now honors injected clipboard readers on any platform; Linux keeps native terminal paste when only the Windows reader is available.
• Checkpoint tests used Windows path syntax on Linux. They now use native paths while still checking trailing separators, stable identities, readable names, and separate agent/project records.
• A bare cd test changed the process working directory and deleted that directory during cleanup. It now restores the original directory and runs in the existing nonparallel collection, protecting unrelated filesystem and process tests.

Clipboard command checks also redirected process stdout into a transcript, making them vulnerable to unrelated test-runner output. They now use an isolated terminal-control sink while retaining exact clipboard bytes and transcript-count assertions. A new concurrent-scope test verifies output isolation and parent-scope restoration.

Validation

• 109 targeted checks passed locally with CI-style ANSI settings.
• Full Release suite with CI-style settings: 1,244 passed on .NET 10 and 1,244 passed on .NET 8.
• All four GitHub CI jobs passed: Windows and Linux on .NET 10 and .NET 8, including three repeated component runs per job. Desktop also builds with zero warnings/errors.