fix(ebuild): use ninja_command() in ebuild build

[agayushh]

ebuild test already preferred a PATH ninja binary and fell back to python -m ninja. ebuild build still hardcoded the module form, so a system ninja install was not enough for the main command.

Fixes #159

Summary

Wire ebuild build to ninja_command() (same helper ebuild test already
uses) and update README.md / demo.md, which still said the pip ninja
package was required because ebuild always invoked python -m ninja.

Type of Change

• feat - New feature
• fix - Bug fix
• docs - Documentation only
• style - Formatting, no code change
• refactor - Code restructuring without behavior change
• test - Add or fix tests
• build - Build system or dependency changes
• ci - CI/CD pipeline changes
• perf - Performance improvement

Changes

• ebuild/cli/commands.py: ebuild build now calls ninja_command() instead of [sys.executable, "-m", "ninja", ...]
• README.md, demo.md: document PATH ninja first, pip module as fallback
• CHANGELOG.md: record the fix
• tests/unit/test_ninja_command.py: unit tests for the helper and for the ebuild build argv

Testing

• Unit tests pass (ctest --test-dir build --output-on-failure)
• Integration tests pass
• Manual testing performed
• New tests added for new functionality

Pre-Submission Checklist

• Code compiles without warnings (-Wall -Wextra -Werror for C)
• All existing tests pass
• New tests added for new functionality
• Documentation updated if API changed
• Commit messages follow the <type>(<scope>): <description> convention
• Branch is rebased on latest master
• This PR links a real issue in this repository with a closing keyword

Related Issue

Fixes #159

Screenshots / Logs

python -m pytest tests/unit/test_ninja_command.py -v --tb=short
4 passed in 0.08s

python -m pytest tests/unit/test_dispatch.py tests/unit/test_doctor.py tests/unit/test_documented_commands_exist.py tests/ebuild/test_build_cli.py tests/unit/test_build_failure_output.py -v --tb=short
33 passed in 0.20s

python -m ruff check ebuild/cli/commands.py tests/unit/test_ninja_command.py
All checks passed

cd examples/hello_world
ebuild build
./_build/hello
Hello from EoS Build System!

Additional Notes

I did not rewrite commands.py line endings. That file is already CRLF in the repository; git diff --check flags \r on newly added lines for that reason.