dcdpr / jp / 25508761275

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chore(tools): Add `start_line`/`end_line` paging to diff tools (#622)

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.

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Signed-off-by: Jean Mertz <git@jeanmertz.com>

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