[REV-2504] Remove retired Opus/Fable 5 desktop promotion

[warp-agent-staging]

Description

Remove the retired pricing-promotion singleton, terminal/agent message-bar actions and telemetry, and onboarding offer propagation/badge override. The offer keeps its ordinary “Recommended” badge.

Keep generic desktop/TUI discount chips, the shared cloud-credit promo pill, ordinary credit banners, and the nullable GraphQL pricing field/client query unchanged. No model catalog, routing, or availability changes.

Linked Issue

REV-2504

Companion server/web cleanup: https://github.com/warpdotdev/warp-server/pull/18976. The server retains the shipped pricing query shape and returns null for the retired message.

Testing

• cargo nextest run -p onboarding --no-fail-fast: all 35 tests pass.
• Focused native app tests for input/message bars, model preferences, and credit state: 376/378 pass. test_decorations_with_multibyte_chars and test_histignorespace_support_in_zsh fail identically on unchanged base 6398d1e96; they are not introduced by this cleanup.
• cargo clippy -p warp -p onboarding --all-targets --tests -- -D warnings: passes, including native compilation of all affected app/test code.
• TUI model/inline-menu tests could not compile: untouched warpui headless/winit delegates lack get_cursor_shape implementations (E0046).
• Wasm compile-only check attempted with wasm32-unknown-unknown, dev-wasm, and release_bundle,gui; blocked in arborium-sysroot because clang is unavailable. Writable Cargo/Rustup caches were used for sandbox permissions.
• Final scoped Rust formatting and git diff --check: pass. Full workspace presubmit and macOS/Windows checks were not run.
• Campaign-only tests were removed with their implementation; existing onboarding and ordinary app behavior tests exercise the retained paths. No visual verification, as requested.

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

CHANGELOG-NONE

[warp-agent-staging]

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