Refactor src/runner.rs: Split module to satisfy ≤400-line rule

[coderabbitai]

Overview

The src/runner.rs module currently contains 434 lines, exceeding the project's ≤400-line rule. This issue tracks the refactoring needed to split this module and improve code cohesion.

Proposed Changes

Extract to runner/process.rs (or similar):

• Ninja process plumbing functionality
• Redaction helpers
• Process-related utilities

Keep in src/runner.rs:

• run() function
• CLI dispatch logic
• High-level generation orchestration

Benefits

• Improved code organization and cohesion
• Adherence to project line count standards
• Better separation of concerns between orchestration and process management

Context

This refactoring was identified during code review of PR #152.

Reference: #152