[ScottishTapWater]

Summary

• Exercise the real executor through public DI in four CoreOnly scenarios: independent versions, unchanged dependencies, custom configuration with symbols, and verification failure before tagging.
• Add CLI process tests for applying a generated plan from a file or stdin, plus CliOnly tests using a stub executor.
• Add apply --output <directory> with --infile, --repository and --include-symbols. Read plans and write results asynchronously; use the plan's configuration and working directory.
• Update the approved README paragraph with the apply command.

Stacked on #24. No publishing or tag pushing.

Verification

• CoreOnly scenarios passed before the CLI tests were written.
• New success-path CLI tests failed with an unrecognised apply command before implementation.
• Full solution: 135 tests passed.
• Formatting and diff checks passed.
• Independent production and test reviews passed.