release: bump all packages to 0.1.1, add PyPI metadata and CHANGELOG

- Add readme field to all three pyproject.toml files (populates PyPI description page)
- Bump versions to 0.1.1 and update behave-core pin in shell/text
- Update Install sections in root, BEHAVE-SHELL, and BEHAVE-TEXT READMEs to lead with pip install from PyPI
- Add CHANGELOG.md at project root (Keep-a-Changelog format)

README.md

@@ -13,15 +13,17 @@ BEHAVE defines a structured observation envelope (`core`) and two domain registr
 
 ## Install
 
-Each package is independently installable. Install `core` first as both registries depend on it.
+All packages are available on PyPI:
 
 ```bash
-pip install -e core/
-pip install -e BEHAVE-SHELL/
-pip install -e BEHAVE-TEXT/
+pip install behave-core
+pip install behave-shell
+pip install behave-text
 ```
 
-For development (includes pytest + ruff):
+`behave-core` is pulled in automatically as a dependency of the registries.
+
+For local development (includes pytest + ruff):
 
 ```bash
 pip install -e "core/[dev]"