Implement FileExecutor for file operations

[bug-ops]

Parent: #239 (Phase A)

Summary

Single FileExecutor handling read, write, edit, glob, and grep operations within a sandboxed path set.

Changes

• crates/zeph-tools/src/file.rs (new): FileExecutor implementing ToolExecutor
• Sandboxed to allowed_paths from config
• Operations: read (with offset/limit), write, edit (string replacement), glob (pattern matching), grep (regex search)

Acceptance criteria

• All 5 file operations functional
• Path sandbox enforced (no traversal outside allowed_paths)
• Unit tests for each operation
• Unit tests for sandbox enforcement