fix(agent): keep New Task stable during task switches

[AnthonyRonning]

Summary

• Keep the top-level New Task button visually stable while an existing Agent task and its follow-up project-trust state load.
• Preserve native disabled behavior and stale-selection fencing, including cross-project switches.
• Keep genuinely unavailable states such as no project, initialization, and new-task creation visibly dimmed.
• Add focused disabled/opacity regression coverage.

Validation

• nix develop .#ci -c ./scripts/ci/frontend.sh — 524 tests passed
• Production frontend build
• git diff --check
• Fresh correctness and UX reviews
• Synthetic merge with open PR feat(agent): improve Agent Mode sidebar behavior #745

End-to-end smoke was intentionally skipped; this is ready for local flicker testing.

Fixes #747

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