feat(billing): single-source usage via main-process relay

[k11kirky]

Problem

The renderer was polling the LLM gateway directly every 30 seconds to fetch usage data, coupling the renderer to the gateway and duplicating polling logic that the UsageMonitorService already handles.

Changes

The UsageMonitorService now caches the most recent usage snapshot (latestUsage) and emits a UsageUpdated event after every successful poll. Two new methods are exposed: getLatest() to retrieve the cached snapshot synchronously, and refreshNow() to trigger an immediate poll.

The usageMonitorRouter exposes these via three new tRPC endpoints: onUsageUpdated (a subscription that streams snapshots as they arrive), getLatest (a query to bootstrap the renderer before the first event), and refresh (a mutation to force an immediate poll). The llmGateway.usage query endpoint has been removed since it is no longer needed.

The useUsage hook in the renderer now subscribes to onUsageUpdated instead of polling on an interval. It bootstraps from getLatest on mount and exposes a refetch that calls the refresh mutation. The window-focus-based refetch interval logic has been removed entirely.

How did you test this?

Three new unit tests were added to service.test.ts:

• Verifies that UsageUpdated is emitted and the snapshot is cached on every successful poll.
• Verifies that UsageUpdated is not emitted and the snapshot remains null when the gateway throws.
• Verifies that refreshNow triggers a fresh poll and returns the resulting snapshot.

Publish to changelog?

No

[k11kirky]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat(billing): single-source usage via main-process relay #2358 👈 (View in Graphite)
• feat(agent): fill Skills/MCP/Rules categories in context breakdown #2357
• feat(sessions): context breakdown popover #2353
• feat(agent): emit per-category context token breakdown #2352
• feat(billing): threshold notification service with persisted dedupe #2351
• feat(billing): always-on Free sidebar bar with reset time #2350
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.