feat: add grep/rg output truncation filter

[bug-ops]

Parent: #717

Problem

grep -r and rg searches can return hundreds of matches. The agent needs representative results, not all of them.

Acceptance Criteria

• Match grep, rg, ripgrep
• If output > 100 lines: keep first 30 + last 10, show total match count
• Config: [tools.filters.grep] with enabled, max_results (default 100)
• Tests
• Expected savings: 60-90%