Enforce animations = false for screen readers

[etraut-openai]

Why

Issue #20489 calls out that animated TUI affordances can be noisy for screen-reader users. Codex already has tui.animations = false as a reduced-motion setting, but some live activity rows render spinner-style prefixes in that mode. These were relatively recent regressions.

We have also regressed this pattern more than once by adding new spinner/shimmer callsites that do not think through the reduced-motion path, so this PR adds a small guardrail while fixing the current surfaces.

What changed

• Omit the live status-row spinner when animations are disabled, so the row starts with stable text like Working (...).
• Render running hook headers without the spinner prefix when animations are disabled, while preserving shimmer/spinner behavior when animations are enabled.
• Centralize TUI activity indicators in tui/src/motion.rs, with explicit reduced-motion choices for hidden prefixes, static bullets, and plain shimmer-text fallbacks.
• Route existing spinner/shimmer callsites through the central motion helper, including exec rows, MCP/web-search/loading rows, hook rows, plugin loading, and onboarding loading text.
• Add a source-scan regression test that rejects direct spinner(...) or shimmer_spans(...) usage outside the central module and primitive definition.
• Add focused coverage that reduced-motion active exec rows are stable, status rows start without a spinner, running hooks omit the spinner, and MCP inventory loading stays stable.
• Update the one affected status-indicator snapshot; the existing detail tree prefix remains unchanged.

Verification

• cargo test -p codex-tui