docs: clarify Claude client setup

[ousamabenyounes]

Summary

Adds a focused README section for setting up pxpipe with Claude clients:

• Claude Code CLI on macOS/Linux
• Claude Code CLI on Windows PowerShell
• Claude Desktop via ANTHROPIC_BASE_URL

Also adds a README regression test so the setup instructions stay discoverable.

Q	A
Issues	Fix #56
Tests	pnpm exec vitest run tests/readme-usage.test.ts; pnpm run typecheck; pnpm test; pnpm run build

Test verification (RED → GREEN)

RED, with the README usage section absent:

FAIL  tests/readme-usage.test.ts > README usage documentation > documents CLI, Windows PowerShell, and Claude Desktop setup
AssertionError: expected -1 to be greater than or equal to 0

Test Files  1 failed (1)
Tests  1 failed (1)

GREEN, with the README usage section restored:

[targeted] pnpm exec vitest run tests/readme-usage.test.ts
Test Files  1 passed (1)
Tests  1 passed (1)

[typecheck] pnpm run typecheck
[test] pnpm test
Test Files  33 passed (33)
Tests  646 passed (646)

[build] pnpm run build
✓ emitted dist/ library modules + declarations
✓ built dist/node.js
✓ version smoke check: --version prints 0.8.0