feat(usage): add codex and agy usage visibility

[StuBehan]

Summary

Adds usage/quota visibility for Codex and Antigravity (agy) to the Usage tab alongside Claude, and reworks the tab into a per-client layout. It now lists each connected client (Claude / Codex / Antigravity) with its plan and shows that client's quota; you can step into the detail pane to scroll it.

Changes

Usage tab → per-client layout (SessionUsage.swift)

• Reworked from a single Claude quota list into a left client list + right detail pane. ↑/↓ switch client; →/Enter step into the pane; once inside ↑/↓ scroll it and ← steps back (Esc hides). A focus ring + footer hints reflect which level has focus. Only clients with data appear.

Codex quota (panel/CodexUsage.swift)

• CodexQuotaProbe reads the newest ~/.codex/sessions rollout's latest payload.rate_limits (primary = 5h, secondary = weekly; used_percent + resets_at + plan_type), mtime-cached, local-only. Rendered as "Codex · ". ChatGPT-plan auth only — API-key users emit none, so it's cleanly absent.

Antigravity (agy) usage (panel/AntigravityUsage.swift)

• AntigravityUsageProbe queries the running agy's local loopback Connect-RPC (GetUserStatus, discovered via lsof; HTTP, no auth/CSRF, 127.0.0.1 only) for per-model quota (remainingFraction → utilization + resetTime) plus plan + credits. Rendered as per-model bars ("Claude Opus 4.6 (Thinking)", "Gemini 3.5 Flash (High)", …) with a credits line, under "Antigravity · ". Only available while agy is running (live RPC, not a persisted file).

Claude plan type (SessionUsage.swift)

• QuotaProbe now also reads subscriptionType from the claudeAiOauth blob it already loads (no new keychain access) → shown as "Claude · ".

Wiring (PanelNav.swift, Panel.swift, build.sh)

• PanelNav: codexQuota/antigravityQuota, usageClientIndex, usageDetailFocused, availableUsageClients. Panel.swift: both probes polled in runQuotaProbe (same cadence + STACKNUDGE_QUOTA_TRACKING toggle as the Anthropic probe); two-level Usage key handling; restored programmatic detail-pane scroll. build.sh: new files added to the swiftc source list.

Testing

• swift build → clean; ./build.sh → full app builds.
• CodexQuotaProbe validated against a real ChatGPT-plan rollout: 5h 1% / weekly 0% / plan plus — matches the raw rate_limits.
• AntigravityUsageProbe validated against the live agy 1.0.6 RPC: plan Pro, per-model bars (Gemini ~19% / Claude 0%), credits 500/50000 + 100/150000 — matches raw GetUserStatus.
• Not exercisable in CI (panel/AX + live agy). Manual check: open Usage → Claude / Codex / Antigravity in the left list with plan labels; ↑/↓ switch; → step into a client (focus ring), ↑/↓ scroll (e.g. agy's per-model bars), ← back.
• No new external network calls — Codex reads a local rollout file, agy hits loopback; the Anthropic probe is unchanged.

Related issues

Antigravity local-RPC usage approach informed by steipete/CodexBar#1178.