fix(cmds/git/diff): preserve POSIX/git contract for programmatic consumers

[YOMXXX]

Summary

`rtk git diff` and `rtk diff` were emitting decorative output that broke `git apply`, `patch`, shell loops over `--name-only`, and exit-code-based predicates. Because the `PreToolUse` hook silently rewrites `git diff` to `rtk git diff` for hooked agents, those decorations silently distorted any patch / script workflow that used to work. Issue #1918 (and its duplicates #1081 / #1869) ask for the POSIX/git contract to be honored.

Two surfaces are fixed:

`rtk git diff` (proxy for `git diff`)

• Detects non-TTY stdout (`std::io::IsTerminal::is_terminal`) and machine-friendly flags (`--name-only`, `--name-status`, `--check`, `--exit-code`, `--raw`, `-z`, `--no-color`, `--numstat`, `--shortstat`, `--dirstat`, `--patch-with-raw`, `--patch-with-stat`, `--output[=…]`).
• When either is true: emit raw `git diff` output verbatim, with byte-for-byte fidelity (`print!` not `println!`) so trailing newlines are preserved for `git apply`.
• Propagate git's actual exit code rather than forcing 0.

`rtk diff` (RTK's GNU-diff-like file compare)

• Returns `0` when files are identical, `1` when they differ, `2` on missing-file / I/O error — the GNU diff exit-code contract.
• The `[ok] Files are identical` status now goes to stderr, so `rtk diff a b > patch.diff` produces an empty patch on identical files (which is what scripts expect) rather than text in stdout.

Reproduction

```bash

rtk git diff round-trip

mkdir -p /tmp/g && cd /tmp/g && git init -q
git config user.email t@t.t && git config user.name t
printf '1\n2\n3\n' > f && git add f && git commit -q -m x
printf '1\n2x\n3\n' > f

rtk git diff > rtk.patch
git stash -q
git apply --check rtk.patch && echo "applies"

Before this PR: 'No valid patches in input'

After this PR: 'applies'

rtk git diff --name-only

Before this PR: 'f\n\n--- Changes ---\n f (...) ...'

After this PR: 'f'

rtk diff exit codes

printf 'a\n' > /tmp/a && printf 'b\n' > /tmp/b
rtk diff /tmp/a /tmp/b; echo "exit=$?"

Before this PR: exit=0 (silently wrong)

After this PR: exit=1 (GNU diff convention)

rtk diff /tmp/nonexistent /tmp/a; echo "exit=$?"

Before this PR: exit=1

After this PR: exit=2 (GNU diff convention)

```

Behavior changes

• Non-TTY `rtk git diff`: no longer prepends a stat summary, no longer wraps in `--- Changes ---`. Output is byte-for-byte `git diff`. The compact path is only taken in interactive (TTY) sessions without machine-readable flags.
• Machine-friendly flags (`--name-only` etc.): same — clean passthrough.
• `rtk diff` exit codes now match GNU `diff`. The "Files are identical" message moves from stdout to stderr.

The compact-diff experience for interactive `rtk git diff` users is unchanged.

Test plan

• `test_has_machine_friendly_diff_flag_detects_name_only`.
• `test_has_machine_friendly_diff_flag_detects_machine_variants` (covers 12 flags).
• `test_has_machine_friendly_diff_flag_ignores_normal_args` (negative).
• `test_has_machine_friendly_diff_flag_detects_output_arg` (covers both `--output=...` and `--output ...`).
• `test_run_returns_0_on_identical_files`.
• `test_run_returns_1_on_different_files`.
• `test_run_returns_2_on_missing_file`.
• All 51 existing diff tests still pass.
• `cargo fmt --all` clean.
• `cargo clippy --all-targets` zero warnings.
• `cargo test --bin rtk -- --test-threads=8` 1908 passed, 0 failed.

Out of scope (deferred to a follow-up)

• `[hooks].exclude_commands` subcommand matching ("git diff" must skip the hook) — issue [hooks] exclude_commands does not cover subcommands (e.g. "git diff") #1919. Wider scope; deserves its own PR.
• TTY detection for `rtk diff` (the GNU-like file compare) is not added — it's a content-transforming command, not a passthrough, so the "non-TTY emits unified diff" semantics don't apply cleanly. Exit-code fix is sufficient for the headline scripting use case.

Fixes #1918, #1869
Refs #1081 (duplicate of #1918)