feat: replace version subcommand with --version flag

[anneschuth]

Summary

• Replaces zad version subcommand with zad --version / zad -V flag
• Follows standard CLI convention (git, docker, uv all use --version)

Test plan

• zad --version prints version
• zad -V prints version
• All 54 tests pass

[claude]

Review

🔴 Critical — backwards-incompatible removal

zad version is a documented public command. Removing it violates the project's explicit backwards-compatibility policy:

No removing CLI commands, options, or positional arguments
Deprecation before removal: add a deprecation warning for at least 2 minor versions before removing anything

Scripts, CI pipelines, and documentation using zad version will silently break.

Required fix: keep the version subcommand for at least 2 minor versions, but have it print a deprecation notice and delegate to the same output:

@app.command(hidden=True)
def version() -> None:
    """[Deprecated] Use `zad --version` instead."""
    import warnings
    typer.echo("Warning: `zad version` is deprecated, use `zad --version` instead.", err=True)
    print(f"zad-cli {__version__}")

Also restore the version entry in test_backwards_compat.py—removing it from the baseline is exactly what that test exists to prevent.

---

🟡 Minor — stray blank line in cli.py at line 109: after removing the version command block there are two consecutive blank lines where one is enough.

Superseded by new review

[claude]

No issues found.

The implementation correctly:

• Adds --version / -V as an eager flag on the root callback
• Keeps the version subcommand with deprecated=True and a deprecation warning on stderr — backwards-compatibility policy satisfied
• Updates _GlobalOptionsGroup._FLAGS so the new flags are hoisted correctly
• Adds tests for both the new flag (test_version_flag) and the deprecated subcommand (test_version_subcommand_deprecated)
• Retains "version" in the test_backwards_compat.py baseline

One extra blank line at cli.py line 121 (after the version command block), but not worth blocking on.

[claude]

🟡 Minor: print() bypasses Typer's output handling. Prefer typer.echo(f"zad-cli {__version__}") for consistency with Typer conventions and to respect NO_COLOR/TERM env vars. (The existing deprecated version command also uses print(), so this is at least internally consistent — but worth cleaning both up together.)

[claude]

Backwards compatibility is maintained — version subcommand is kept (deprecated) and remains in EXPECTED_COMMANDS in the backwards-compat test. The hoisting of --version/-V in _GlobalOptionsGroup._FLAGS is correct and consistent with how --verbose is handled. is_eager=True + raise typer.Exit() is the correct Typer pattern.

One minor nit left inline (print vs typer.echo), otherwise this is clean.