Implement parsing for -Dkey=@file and @@ escaping

[Matographo]

Summary

Load variable values from files and treat @@ as a literal @ escape sequence.

Why

File-backed values are required for larger templates and secrets-free content injection, while @@ prevents ambiguous parsing.

Acceptance Criteria

• File-backed values load correctly
• @@ escapes @ without triggering file reads
• I/O failures return the expected error class

Dependencies

• Depends on Implement the variable store with deterministic last-write-wins behavior #6
• Depends on Implement parsing for -Dkey=value #7