Production-hardening: fix README API mismatch, add buildable PlatformIO config, real CI

[SuperInstance]

Summary

• README's entire Quick Start didn't match the real API — wrong constructor arity, wrong begin() argument order, three fabricated sensor-registration method names that don't exist. Fixed to match src/openconstruct-esp32.h exactly.
• Fixed the one example's header include (wrong filename casing).
• Added a real platformio.ini — none existed despite the README's own PlatformIO install instructions; a stale .gitignore line was also actively excluding it from ever being committed.
• Found and fixed real compile errors during actual build verification: String::toLowerCase() returns void in this Arduino core, but the code was assigning its result — this would not have compiled at all.
• Added real CI (.github/workflows/ci.yml) running pio run -e esp32dev and compiling the test suite.
• Verified all 4 README sibling-repo cross-links actually exist.

Verification

• pio run -e esp32dev: SUCCESS (real ESP32 toolchain build, not just code review)
• pio test -e test --without-uploading --without-testing: compiles clean
• Verified independently, commit by commit, against the real header before trusting each fix

🤖 Generated with a multi-agent hardening pass (kimi), independently verified before opening this PR.