feat: add find output truncation filter

[bug-ops]

Parent: #717

Problem

find can return thousands of lines. The agent rarely needs all of them.

Acceptance Criteria

• Match find command
• If output > 50 lines: keep first 20 + last 10, show total count
• Config: [tools.filters.find] with enabled, max_results (default 50)
• Tests
• Expected savings: 70-90%