Add accessibility baseline: no-color and reduced motion

[raiseCatError]

Implements the delivery half of #170; the audit and acceptance criteria are in docs/accessibility/baseline.md.

Stacked on #218 (feature/167-form-controls), on #217, on #203. Merge order: #203, #217, #218, then this PR.

What changed

• presentation/capabilities.ts: colorLevel(). NO_COLOR, TERM=dumb or NMSH_COLOR=none turn NMSh-owned color off. Without an explicit signal, behavior is unchanged (truecolor).
• ui/palette foreground()/background() honor it. The remaining raw truecolor escapes in the transcript presenter and paste atoms now go through them.
• NMSH_REDUCED_MOTION=1 (implied by deterministic presentation) freezes the shimmer/activity phase and the welcome blink. Measured durations are unaffected.
• Docs record the acceptance criteria, audit result, known gaps and the screen-reader limitation.

Not included

• 256/16-color downconversion (left to Chroma integration: shared color roles across NMSh UI #171) and a persisted reduced-motion setting (public config schema; left to Settings v2).
• Command-row background bands remain a color-only cue under NO_COLOR; recorded as a known gap.

Verification

tests/accessibilityBaseline.test.ts; full suite 636/636, typecheck, build and git diff --check clean.