docs: add design doc for unified cost calculation

Shared pricing module replacing dual cost calculation systems
(jsonl.ts hardcoded + sessionAnalyzer.ts LiteLLM) with a single
src/shared/utils/pricing.ts used by both main and renderer processes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/plans/2026-02-23-unify-cost-calculation-design.md

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+# Unify Cost Calculation — Design Document
+
+**Date:** 2026-02-23
+**Branch:** `feat/unify-cost-calculation`
+**Related:** PR #60 (Session Analysis Report), PR #65 (Cost Calculation Metric), Issue #72 (Plan Usage Tracking)
+
+## Problem
+
+Cost calculation exists in two places with different pricing data and logic:
+
+1. **Main process** (`src/main/utils/jsonl.ts`): Uses LiteLLM-sourced `pricing.json` (206 models, tiered 200k-token pricing). Populates `SessionMetrics.costUsd` for the chat UI.
+2. **Renderer** (`src/renderer/utils/sessionAnalyzer.ts`): Uses a hardcoded 6-model pricing table with no tiered pricing. Generates per-model cost breakdown for the Session Report.
+
+The two systems can produce different cost numbers for the same session and will drift further as models change.
+
+## Solution
+
+Create a single shared pricing module that both processes import.
+
+### New Module: `src/shared/utils/pricing.ts`
+
+**Exports:**
+
+| Export | Description |
+|--------|-------------|
+| `ModelPricing` | Interface for per-model rates (input, output, cache read, cache creation, plus tiered variants) |
+| `getPricing(modelName: string): ModelPricing \| null` | Model lookup: exact match, lowercase, case-insensitive scan |
+| `calculateTieredCost(tokens, baseRate, tieredRate?): number` | Applies 200k-token tier threshold |
+| `calculateMessageCost(model, inputTokens, outputTokens, cacheReadTokens, cacheCreationTokens): number` | Computes cost for a single API call |
+
+**Pricing data:** Static `import pricingData from '../../../resources/pricing.json'` with `resolveJsonModule: true`. Replaces `fs.readFileSync` runtime loading.
+
+### Consumer Changes
+
+**`src/main/utils/jsonl.ts`:**
+- Remove: `ModelPricing` interface, `loadPricingData()`, `calculateTieredCost()`, `getPricing()`, `fs`/`path` imports
+- Keep: `calculateMetrics()` function
+- Change: inline cost loop body → call `calculateMessageCost()` from `@shared/utils/pricing`
+
+**`src/renderer/utils/sessionAnalyzer.ts`:**
+- Remove: `MODEL_PRICING` table (~40 lines), `DEFAULT_PRICING`, local `getPricing()`, local `costUsd()`
+- Change: calls at lines 476 and 900 → `calculateMessageCost()` from `@shared/utils/pricing`
+
+**Tests:**
+- `test/main/utils/costCalculation.test.ts` → update to test shared module functions
+- `test/renderer/utils/sessionAnalyzer.test.ts` → mock `@shared/utils/pricing` instead of local functions
+- New `test/shared/utils/pricing.test.ts` for the shared module
+
+### Pricing JSON Import Strategy
+
+- `pricing.json` stays in `resources/` for Electron's `extraResources` packaging
+- Both Vite (renderer) and electron-vite (main) resolve the JSON import at compile time
+- Remove the `fs.readFileSync` dev/prod path logic from `jsonl.ts`
+
+### Fallback Behavior
+
+- `getPricing()` returns `null` for unknown models
+- `calculateMessageCost()` returns `0` for unknown models (matches current `jsonl.ts` behavior)
+- Session analyzer callers can apply a default if needed
+
+### What Changes for Users
+
+- Report costs become more accurate (tiered pricing, 206 models instead of 6)
+- Cost numbers between chat view and Session Report now agree exactly
+- No UI changes — same components, same layout
+
+## Out of Scope
+
+- Plan usage tracking (see Issue #72 — pending community feedback)
+- New UI surfaces for cost display
+- Changes to the `costFormatting.ts` shared utility