feat: add respect_gitignore config option

[xiaocang]

What does this PR do?

Adds a new respect_gitignore config option that controls whether .gitignore rules are respected when searching files.

How did you verify your code works?

• Tested with respect_gitignore: false and confirmed gitignored files (e.g., node_modules/, build artifacts) appear in glob/grep/ls results
• Tested with default config (no setting or respect_gitignore: true) and confirmed gitignored files are excluded as before

Fixed https://github.com/anomalyco/opencode/issues/7969