docs: add portable harness onboarding

[SarthakWade]

Summary

• make the canonical browser-use skill auto-discoverable through the Claude skill path
• add project-scoped MCP configuration for three supported clients through one repository-relative stdio launcher
• replace the website's personal SSH target with the checked-in portable Cursor configuration
• document native, installed, and Docker MCP setup paths
• validate config parity, skill linkage, launcher behavior, and website provenance in the web lint gate

Security

• keeps MCP local on stdio without a TCP listener or Chromium debug port
• rejects launcher arguments and requires absolute executable overrides
• preserves the canonical skill as a single source through a checked-in symlink

Verification

• pnpm test including 44/44 protocol tests, MCP integration, and release checksum tests
• pnpm test:runtime
• pnpm --filter @headless/web lint
• pnpm --filter @headless/web build
• pnpm test:e2e:linux
• pnpm test:e2e:mac
• native release build and real MCP initialize plus tools/list through the launcher
• shell syntax, QA evidence checksums, git diff --check, and Prettier

Closes #33
Closes #52