fix(diff): report modified-only diffs and follow diff exit convention

[hgunduzoglu]

Summary

Fixes #2364 — rtk diff reported differing YAML/JSON files as [ok] Files are identical (exit 0), hiding real changes from agents comparing config files.

Root cause

Not extension-related: compute_diff classifies similar lines (character-set similarity > 0.5, e.g. a: 1 vs a: 2) as modified, but the identical check in run() only tested added == 0 && removed == 0, ignoring the modified count. Structured files like YAML/JSON trigger this because their changed lines stay near-identical character-wise; the issue's .txt repro worked only because x vs y has zero similarity and lands in added/removed.

Changes

• src/cmds/git/diff_cmd.rs — extracted render logic into a pure, testable render_file_diff(); the identical check now uses diff.changes.is_empty(); returns the diff-convention exit code (0 = identical, 1 = differing), also fixing the secondary observation in the issue.
• src/main.rs — Commands::Diff propagates the exit code (same Result<i32> pattern as go_cmd / core::runner).
• scripts/test-all.sh — diff smoke test updated for the exit convention (identical → ok, differing → non-zero + output contains changes).
• README.md — exit semantics documented in the command list.

Testing

• TDD: 4 new regression tests written first (red on the old logic), covering the exact repro cases from the issue (a: 1/a: 2 YAML, {"a": 1}/{"a": 2} JSON, identical files, added/removed .txt).
• cargo fmt --all --check && cargo clippy --all-targets && cargo test — clean, 2154 passed.
• Manual verification of all repro scenarios from the issue with the built binary (output + exit codes match /usr/bin/diff semantics).

Behavior change note

rtk diff previously always exited 0. It now exits 1 when files differ, matching diff convention — as requested in the issue's "Expected" section, and required by scripts using diff && echo same semantics under the hook rewrite.

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[aeppling]

Hey @hgunduzoglu , PR look clean and solve the issue

Follow-up

Merging this for the fix, but git diff will need some work to avoid breaking any signal, for now this is kind of tricky and should be cleaned

LGTM

Thanks for contributing !