feat(billing): add posthog code token spend analysis banner

[pauldambra]

Problem

PostHog Code users on the Plan & usage page have no way to see how their LLM token spend breaks down. The data exists in PostHog's LLM analytics — we just haven't surfaced it back inside the app. This PR adds an in-app banner that runs the analysis on demand and prints the result inline, doubling as an upsell for PostHog's LLM analytics product.

Changes

• New TokenSpendAnalysisBanner rendered at the top of Settings -> Plan & usage. Idle state is an upsell callout with an "Analyse my spend" button; click runs the analysis, swaps to inline tables and heuristic suggestions on completion.
• New useTokenSpendAnalysis hook (renderer-side useState/useEffect pattern matching other cloud-API callers) and a new getPostHogCodeSpendAnalysis(days) method on PostHogAPIClient that hits GET /api/llm_analytics/posthog_code_spend/.
• Response types in apps/code/src/renderer/features/billing/types/spend-analysis.ts, mirroring the backend serializer.
• Footer links: PostHog LLM analytics docs and the exploring-llm-costs skill in the skill store.
• Gated behind posthog-code-spend-analysis (always on in dev). The BILLING_FLAG that gates Plan & usage itself is also forced on in dev so the section is reachable without per-developer flag setup.

Backend endpoint: PostHog/posthog#59017.

Screenshots

Banner idle state — Settings → Plan & usage:

How did you test this code?

I'm an agent. I did not perform manual UI testing — the screenshot above is from the human author's local run.

• pnpm --filter=@posthog/code typecheck passes on the rebuilt branch.
• pnpm biome check --write --unsafe clean on the touched files.

The banner is reachable in dev at Cmd+, → Plan & usage.

Publish to changelog?

no — backend dependency lands first; user-facing announcement comes later.

🤖 Agent context

Authored with PostHog Code (Claude Opus 4.7).

• Approaches considered and rejected:
  • Letting an agent task handle the analysis (background task with results in side panel) — adds friction for a one-shot upsell view.
  • Calling MCP directly from the renderer — wrong scope; MCP is per-agent auth.
  • Querying the customer's own PostHog project's HogQL — wrong project; the data lives in PostHog's internal cloud analytics team.
• Chose a fixed backend report + inline tables. Faster, deterministic, no LLM in the loop for data fetching.
• Suggestions are hardcoded heuristics (Bash share, no-tool share, top-trace share). Easy to swap for an LLM-generated narrative later if useful.

[greptile-apps]

Prompt To Fix All With AI

Fix the following 3 code review issues. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 3
apps/code/src/renderer/features/billing/hooks/useTokenSpendAnalysis.ts:28-33
Refresh errors are silently swallowed after the first successful load. Because `data` is never cleared on failure, the banner always takes the `if (data)` branch in `TokenSpendAnalysisBanner`, so the `if (error)` branch is unreachable once data has been fetched. When a refresh fails the user sees the stale previous result with no indication anything went wrong.

```suggestion
      const result = await client.getPostHogCodeSpendAnalysis(days);
      setData(result);
      setError(null);
    } catch (err) {
      const message = err instanceof Error ? err.message : "Unknown error";
      log.warn("Failed to fetch spend analysis", { error: message });
      setData(null);
      setError(message);
```

### Issue 2 of 3
apps/code/src/renderer/features/billing/components/TokenSpendAnalysisBanner.tsx:23-24
The `amount < 1` branch is a strict subset of `amount < 100` and performs the exact same operation (`toFixed(2)`), so it is dead code that adds no distinction. This violates the "no superfluous parts" simplicity rule.

```suggestion
  if (amount < 100) return `$${amount.toFixed(2)}`;
```

### Issue 3 of 3
apps/code/src/renderer/features/billing/types/spend-analysis.ts:22-28
`SpendAnalysisModelRow` is defined and `by_model` is included in `SpendAnalysisResponse`, but the component never renders a model breakdown table. The data is fetched over the wire and typed, yet discarded. Either a `ModelTable` is missing from the banner, or the interface and field should be dropped to satisfy the "no superfluous parts" rule.

_{Reviews (1): Last reviewed commit: "feat(billing): add posthog code token sp..." | Re-trigger Greptile}

[joshsny]

LGTM - wondering if it should be an MCP tool and we send them into the chat, probably not since they want to look at this when they ran out of $ so that would be frustrating