grep tool: prefer ripgrep (rg) when available

[stefanthearchitect]

Context

The grep tool shells out to grep -rEIn (internal/tools/exec.go, grepTool.Execute). ripgrep (rg) is dramatically faster on large repos and respects .gitignore by default, which usually matches what a coding agent wants to search.

The task

When rg is on PATH, use it; otherwise fall back to the current grep invocation unchanged. Keep the tool's output shape (path:line:match) identical so nothing downstream changes.

Notes / constraints

• Fallback must be exact — a machine without rg behaves exactly as today.
• Keep it project-agnostic: detect the binary, don't assume a repo type.

Acceptance

• With rg present, searches use it; output format unchanged.
• Without it, behavior is byte-identical to now.
• Tests for both paths. Coverage ≥90%.