feat(runner): watch mode

[CalvinAllen]

Description

Automatically re-run tasks when files change.

Proposed Syntax

dev:
  watch:
    paths: [src/**/*.rs]
    ignore: [target/]
  cmd: cargo build

Acceptance Criteria

• Watch specified file patterns
• Ignore specified patterns
• Debounce rapid changes
• Clear output option
• Graceful shutdown (Ctrl+C)

Notes

Consider using notify crate for cross-platform file watching.