fix(grep): honor head_limit values above 100

[T0mSIlver]

Problem

The schema says head_limit limits output to the first N entries. The clamp was:

limit = 100
if head_limit is not None:
    if head_limit < limit and head_limit > 0:
        limit = head_limit

Because of the head_limit < limit guard, any value >= 100 was silently ignored and the cap stayed at 100 — the model could never raise the limit. (The test even passes head_limit: 200 and gets 100.) The schema also claimed unspecified means "shows all results from ripgrep", which was never true.

Fix

• Honor any positive head_limit, including values above 100.
• Keep a 100-line safety cap when head_limit is omitted, and update the schema description to state that accurately.
• Add test_grep_head_limit (hermetic, 250-line file) asserting head_limit=150 is honored and an omitted value falls back to 100. This test fails against the old clamp.

Paper reference

The paper defines head_limit as "Limit output to the first N lines or entries" (FastContext paper (arXiv:2606.14066), Appendix E "FastContext Explorer: Tool Schemas", p. 19), confirming N must be honored. The paper specifies no hard cap and no "shows all results" behavior — the 100-line default applied when head_limit is omitted is an implementation choice (judgment call), not from the paper.