docs(mobile): clarify approval tint contrast

[rynfar]

Problem

The Live Activity tint comment said every light-background status used the web palette's -600 shade, but approval intentionally uses amber-700 so it retains 4.5:1 contrast on light notification surfaces.

Fix

Document the amber-700 accessibility exception. Runtime colors and behavior are unchanged.

Verification

• vp fmt --check apps/mobile/src/widgets/AgentActivity.tsx — passed.
• git diff --check — passed.
• Commit hook formatting — passed.

Generated with GPT-5.6-sol via Prime Agent.

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[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	13.6 KiB	13.7 KiB	+177 B (+1.3%)	15.1 KiB	✅
Codex	Thread snapshot wire	6.9 KiB	6.9 KiB	−9 B (−0.1%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.6 KiB	6.8 KiB	+186 B (+2.7%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	57.2 KiB	58.7 KiB	+1.5 KiB (+2.6%)	66.4 KiB	✅
Codex	Live turn messages	10	11	+1 (+10.0%)	21	✅
Claude	Total thread wire	13.6 KiB	13.6 KiB	0 B (0.0%)	15.1 KiB	✅
Claude	Thread snapshot wire	6.9 KiB	6.9 KiB	−8 B (−0.1%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.6 KiB	6.6 KiB	+8 B (+0.1%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	58.1 KiB	58.1 KiB	0 B (0.0%)	66.4 KiB	✅
Claude	Live turn messages	10	10	0 (0.0%)	21	✅

Baseline: 9c2a08d · PR result: db54e8e · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 109.5 KiB
• Claude decoded thread snapshot: 110.2 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.