Settings: Providers and Default providers polish found

Found while visually checking Settings › Providers and Accounts › Default providers (Task 7418): 1. Default providers lives under Accounts while the provider capability cards live under Providers; put the allowlist grid in the Providers category (or link each way) so one place answers "which providers, which auth kinds, what they can do". 2. Clicking a settings category leaves keyboard focus on the rail; move focus to the opened category's column so arrow/page keys scroll it without extra Tabs. 3. Gemini's gap "The repository's own .gemini/ and .agents/ directories" is a seven-line paragraph; trim the catalogue consequence to the operator-relevant effect (one or two sentences) and leave mechanics to docs. 4. Antigravity lists 15 gaps; group harness-level gaps shared by several engines (guards, subagents, hooks) under one heading so per-provider differences stand out. Out of scope: runtime switch semantics, the acp_runtime knob and dispatch. Done means: - Each item is done or declined with a one-line reason, with 1440px and 390px screenshots of the affected surfaces. Required checks: ./dc check full

PR #6039 merged: harness gaps counted apart in Settings › Providers, rail click focuses the category column, Gemini gap trimmed. Item 1 was already shipped (7296/7418). Quick gate and changed-area tests green; visual check at 1440/390px done. Draft follow-up #7497.