chore(tools): Add start_line/end_line paging to diff tools

[JeanMertz]

Both git_diff_file and git_diff_commit previously had only one escape hatch when a diff exceeded the 500-line truncation cap: the pattern grep parameter. That left no way to inspect a specific region of a large diff without already knowing what to search for.

This change adds start_line and end_line parameters to both tools, mirroring the interface of fs_read_file. When either bound is supplied, the diff is sliced to that 1-based output-line window before any further processing, and the truncation cap is bypassed entirely — the caller owns their window size. Slice markers are injected last (... (starting from line #N) ... / ... (truncated after line #M) ...) so they survive an intermediate pattern grep.

The two features compose: when both a range and a pattern are given, the diff is sliced first and pattern greps within that slice. This lets the model page to a known region and then narrow further.

Three new helpers in diff_filter.rs carry the implementation: validate_line_range (static cross-cut checks), slice_diff (extracts the line window), and add_slice_markers (appends the context markers). The truncation note now surfaces both escape hatches so the model knows to use start_line/end_line when it hits the cap.

Tool schemas and the git-reading skill description are updated to document the new parameters and include paging/compose examples.