docs: document RTK runtime and output boundaries

[mmmihaeel]

Summary

• turn the README into a portfolio-grade product overview
• document architecture, configuration, installation, compatibility, security, contribution, and troubleshooting boundaries
• add an honest v0.1.0 changelog and release contract

Verification

• 49 of 50 public tests pass on Ubuntu/WSL2; one intentional skip
• GitHub Actions Ubuntu baseline is green
• JSON manifests parse and hook executable modes are preserved
• Prettier, Markdown lint, local-link checks, git diff --check, English-only docs, and legacy-identity scans pass

Scope

Documentation only. Product code, manifests, tests, and CI behavior are unchanged.