feat(vault-config): ui.reduced_motion key (reserved groundwork)

[UnbreakableMJ]

What

Adds ui.reduced_motion, completing the PRD §7.1 config registry
(clipboard.*, agent.*, sync.*, now ui.*). It's the accessibility
preference to suppress animated TUI elements (spinner / lock-countdown).

Honest scope: it's reserved groundwork

Exploration confirmed the TUI has zero animation today — toasts are a static
Option<String> (no expiry timer), there are no blink modifiers, and the event
loop is purely event-driven (the only timer is the OSC52 clipboard-clear). So
the key has nothing to act on yet. Per the decision taken with the user, we
ship it inert now: record/validate the preference and plumb the flag, ready
for the future spinner / lock-countdown to honor without re-plumbing. No
visible effect today.

How

• vault-config: [ui] table + ui.reduced_motion (bool) — get/set/
  unset + reduced_motion() accessor (reuses parse_bool). It's a
  TUI-rendering preference, so it is not relayed via agent_args (the agent
  never renders), unlike the agent-side keys.
• vault-tui: App.reduced_motion, populated from config::load() in
  load_app. (A bare pub field — rustc doesn't flag it as dead, so no
  attribute needed; a future renderer reads it.)

Tests

• vault-config: ui.reduced_motion round-trip (true/false), rejects a
  non-boolean, and is asserted never to appear as an agent flag.

Local CI-exact gates all green: fmt, fresh isolated clippy -D warnings,
cargo test --workspace --all-targets under RUSTFLAGS=-D warnings,
cargo deny check (no new deps), both headless builds.

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