Follow-ups for imperative command gating

[aidandaly24]

Follow-ups to #2396:

• Consolidate Gateway mutation availability under the parent imperative-command gate and remove the legacy mutation-only setting: refactor(gateway): remove the separate mutation feature flag #2402.
• Inline the imperative screen-rendering test helper so test configuration is explicit at each call site: test(tui): inline imperative screen configuration #2401.
• Close TUI bypasses: hide and block standalone mutation/execution actions when disabled, restore them when enabled, and preserve project workflows and read-only resource inspection: fix(tui): gate imperative mutations and execution screens #2403.

All three PRs are drafts against refactor. The renderer refactor overlaps tests
updated by the two fixes and should be rebased after those changes merge.