Warn when grep file scan hits limit

[gunnlace]

Summary

GrepTool._walk() stops after collecting 5000 files, but the previous output did not tell users when the scan was truncated. This could make No matches found. misleading because only part of the tree was searched.

This PR:

• Returns a truncation flag from _walk()
• Appends a warning when grep results may be incomplete
• Adds a regression test for truncated file scans
• Updates the README test count from 86 to 87

Tests

• uv run --with ruff ruff check
• uv run --with pytest pytest tests/ -q
• uv run python -m compileall corecoder