Refresh README and add GitHub-only installation

[rajanbor]

Outcome

• redesigns the repository landing page around MAP value, workflow, commands, and structure
• clearly states that the npm package is not published
• replaces npm instructions with installation from the GitHub checkout
• adds a safe, idempotent scripts/install.sh that builds and links the CLI
• refuses to overwrite an unrelated map executable
• aligns Getting Started and the CLI README
• pins the workspace package-manager version for reproducible Corepack builds
• tests the GitHub installer in CI

Verification

• clean installer run into an isolated bin directory
• repeated installer run succeeds
• collision with an unrelated executable is rejected before build
• installed map --version reports 0.5.1
• bash -n scripts/install.sh
• pnpm lint, pnpm typecheck, pnpm test, pnpm build
• pnpm audit --audit-level=low
• schema and registry validation

Closes #101