feat(usage): provider enablement settings, usage-source registry, and multi-provider usage modal #1102

Motivation

The usage modal, header indicator, and exhaustion-driven switch suggestions are hardcoded to Claude + Codex (UsageProviderKind, usageService.ts), while five provider kinds are registered. Users who never use a provider (e.g. Grok) still see it everywhere; z.ai GLM Coding Plan usage (via OpenCode) has no surface at all.

Spec: docs/superpowers/specs/2026-09-20-provider-usage-enablement-design.md (approved).

Intended behavior

• New main-owned provider enablement state per agent kind, defaulting to PATH auto-detection; explicit user toggles pin the value; Reset restores detection.
• New Providers settings category: toggle + detection hint per provider; OpenCode row gains a usage-source dropdown (none | zai, extensible).
• A disabled provider disappears from new-agent pickers, single/bulk provider switch menus, fleet bulk actions, and the usage modal/header. Running sessions are untouched.
• Generalize usage to a UsageSourceId registry (claude | codex | grok | opencode:zai) in src/main/usage/; usageService composes the active source list from settings, preserving per-source failure isolation, 30s TTL, in-flight coalescing.
• Usage modal redesign: provider rail + detail pane, per-source loading skeleton from the known active-source list, dynamic subtitle, keyboard-navigable rail, empty state linking to Providers settings.

This issue covers Phase 1 only (Claude + Codex live). Grok and z.ai readers are separate issues; their sources ship dark until those land.

Acceptance criteria

• Enablement state with detection defaults, user override pinning, reset — unit tested.
• All listed surfaces filter on enablement (exhaustive vs AGENT_PROVIDER_KINDS consumers, contract-test enforced).
• getUsageSnapshot returns composed active sources; disabled ⇒ absent, never an error row.
• Modal renders N sections in rail+detail layout with real loading skeleton; keyboard nav; empty state.
• Existing behavior preserved for Claude/Codex rows, header polling, /usage shortcut, exhaustion suggestions.
• npm run check-relevant test tiers pass.

Refs the spec above; implements Phase 1.