fix(diff): exit 2 on unreadable operand per POSIX diff convention

[hgunduzoglu]

Summary

Completes #2446. rtk diff now follows the full POSIX diff exit-code convention:

Scenario	Before #2394	After #2394 (merged)	This PR
Files identical	0 ✓	0 ✓	0 ✓
Files differ	0 ✗	1 ✓	1 ✓
Unreadable operand (missing file, etc.)	1 ✗	1 ✗	2 ✓

The differing-files row was resolved by #2394 (merged). This PR closes the remaining gap: I/O errors.

Root cause

run() read both operands with ?. A read failure propagated up to main()'s error handler, which returns exit 1 for any Err — indistinguishable from "files differ". So a typo'd path or a deleted file was silently reported as a difference (exit 1) instead of trouble (exit 2).

Fix

Read the operands explicitly via a small read_operands helper: on any I/O error it prints a diff-style message to stderr and returns exit code 2, mirroring system diff. No change to the identical/differ paths.

$ rtk diff one.yaml missing.yaml; echo "exit:$?"
rtk diff: missing.yaml: No such file or directory (os error 2)
exit:2                  # was: exit:1

$ diff one.yaml missing.yaml; echo "exit:$?"   # system diff, for parity
diff: missing.yaml: No such file or directory
exit:2

Testing

• 4 new unit tests: first-operand-missing → 2, second-operand-missing → 2, both-readable → Ok, and exit-2 distinct from 0/1. Paths built from CARGO_MANIFEST_DIR so they don't depend on cwd.
• cargo fmt --all --check && cargo clippy --all-targets && cargo test — clean, 2203 passed.
• Manual verification against system diff for all three exit codes (0/1/2).

No output-format change, so token savings are unaffected (the error path prints a one-line stderr message, same as before).