Implement parsing for -Dkey=value

[Matographo]

Summary

Support direct variable injection from the CLI.

Why

-Dkey=value is the minimum viable variable input mechanism for nearly all CLI usage.

Deliverables

• CLI parsing for -Dkey=value
• Validation of malformed definitions
• Integration into the variable store

Acceptance Criteria

• -Dname=Alice sets name correctly
• Multiple -D flags are processed in order
• Invalid forms return a CLI or configuration error

Dependencies

• Depends on Specify CLI behavior, inspection commands, and exit codes #2
• Depends on Implement the variable store with deterministic last-write-wins behavior #6