Hide current iOS Agent GUI

[azooz2003-bit]

Summary

• Keep the Agent GUI implementation compiled and testable.
• Disable its current iOS product exposure through one hard-false availability boundary.
• Hide the live transcript, toolbar toggle, transcript demo route, and transcript density setting.
• Add unit and UI behavior coverage for the hidden state.

This PR is intentionally stacked on #8665 because that branch contains the Agent GUI implementation.

Testing

• swift test --package-path Packages/iOS/CmuxMobileShell --filter AgentGUIAvailabilityTests (7 passed)
• git diff --check
• Tagged iOS build and focused hosted UI test pending.

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Summary by cubic

Hides the current iOS Agent GUI behind a hard‑false product boundary and removes the portal view‑hierarchy mutation tracker to avoid a launch crash while keeping split‑divider hit‑testing reliable. Previously debug/settings surfaces could surface the Agent UI and portal caches used swizzled mutation tracking; now availability resolves to nil and cache invalidation uses root subview identity plus targeted subview observation.

• Adds AgentGUIAvailability.isUserInterfaceExposed = false and deriveForUserInterface(...); WorkspaceDetailView now uses the gated resolver. Gates the transcript demo preview route, toolbar toggle, Developer > Transcript Demo, and hides the Transcript Density picker in Settings; root view gating updated to require availability.
• Adds unit and UI tests asserting all Agent GUI entry points remain unavailable.
• Removes the portal mutation tracker and related tests; PortalSplitDividerCacheInvalidator drops AppKit swizzling, observes frame/bounds/hidden and subview changes on targeted views; BrowserWindowPortal/TerminalWindowPortal validate caches using root subview identity. Project references cleaned up.
• No migration actions required.

^{Written for commit 7cf71e7. Summary will update on new commits.}

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