docs: replace bun references with pnpm

Replace all bun command references with pnpm equivalents:
- Update installation instructions in README.md
- Update example run commands in examples/README.md
- Update benchmark commands in benchmarks/REPORT.ja.md
- Change Nix flake description from "Bun" to "pnpm"

This completes the migration from bun to pnpm as the project's
package manager.

Author: ryoppippi

README.md

@@ -19,8 +19,8 @@ npm install @stackone/ai
 # Using yarn
 yarn add @stackone/ai
 
-# Using bun
-bun add @stackone/ai
+# Using pnpm
+pnpm add @stackone/ai
 ```
 
 ### Optional: AI SDK Integration
@@ -34,8 +34,8 @@ npm install ai
 # Using yarn
 yarn add ai
 
-# Using bun
-bun add ai
+# Using pnpm
+pnpm add ai
 ```
 
 ## Development Environment
@@ -53,7 +53,7 @@ echo "use flake" > .envrc
 direnv allow
 ```
 
-The flake provides all necessary development dependencies including Node.js, Bun, and other build tools.
+The flake provides all necessary development dependencies including Node.js, pnpm, and other build tools.
 
 ## Integrations
 

examples/README.md

@@ -70,7 +70,7 @@ export const ACCOUNT_IDS = {
 
 ```bash
 # Run a specific example
-bun run examples/ai-sdk-integration.ts
+pnpm run examples/ai-sdk-integration.ts
 
 # Or with Node.js
 npx tsx examples/ai-sdk-integration.ts
@@ -80,7 +80,7 @@ npx tsx examples/ai-sdk-integration.ts
 
 ```bash
 # Test all examples
-bun test examples/
+pnpm test examples/
 ```
 
 ## Examples Overview
@@ -273,13 +273,13 @@ The examples include a comprehensive test suite:
 
 ```bash
 # Run all example tests
-bun test examples/
+pnpm test examples/
 
 # Run with verbose output
-bun test examples/ --verbose
+pnpm test examples/ --verbose
 
 # Run specific test
-bun test examples/examples.spec.ts
+pnpm test examples/examples.spec.ts
 ```
 
 ## Troubleshooting