feat: Add usage limit detection and upgrade prompt

[charlesvien]

Problem

NOTE TO REVIEWER: You might not be able to test this without enabling the feature flag in a SEPERATE org, feel free to yolo stamp.

Free-tier users have no visibility into usage limits until requests start failing silently.

Changes

1. Extract useUsage hook from PlanUsageSettings into shared billing/hooks for reuse
2. Add useUsageLimitDetection hook to detect when free users exceed sustained/burst limits
3. Add UsageLimitModal with context-aware messaging (mid-task vs idle)
4. Add SidebarUsageBar showing usage percentage and upgrade link
5. Gate all billing UI behind posthog-code-billing feature flag
6. Rremove assertBillingEnabled() from all store actions. The callers own that responsibility.

How did you test this?

Manually

[charlesvien]

• feat: Make plan approval options contextual #1794
• feat: Add fullscreen expand to plan preview #1793
• fix: Misc bug and nit fixes #1792
• feat: Add usage limit detection and upgrade prompt #1791 👈 (View in Graphite)
• main

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[greptile-apps]

Prompt To Fix All With AI

This is a comment left during a code review.
Path: apps/code/src/renderer/features/billing/stores/usageLimitStore.ts
Line: 22
Comment:
**Context cleared before close animation completes**`hide()` sets `context: null` at the same time as `isOpen: false`. Radix UI's `Dialog` plays an exit animation while its content is still mounted, so the description briefly switches from the "mid-task" copy to the "idle" copy mid-animation. Keeping `context` alive until the dialog is fully closed prevents the visual flicker.
```suggestion hide: () => set({ isOpen: false }),```
How can I resolve this? If you propose a fix, please make it concise.
---
This is a comment left during a code review.
Path: apps/code/src/renderer/components/MainLayout.tsx
Line: 44
Comment:
**Modal can be triggered before it is mounted**`useUsageLimitDetection()` runs unconditionally while `<UsageLimitModal />` is only rendered when `billingEnabled` is true. A mid-session flag flip from `true → false` leaves `isOpen: true` stranded in the store with no modal to render it. Consider hoisting the flag check to the call site or calling `hide()` when `billingEnabled` turns `false`.
How can I resolve this? If you propose a fix, please make it concise.

_{Reviews (1): Last reviewed commit: "Update seatStore.test.ts" | Re-trigger Greptile}

[jonathanlab]

LGTM, nice work 🚢