feat(cli): add commute subcommand

[hzgpmc]

Summary

• add qsym commute <A> <B> to the CLI
• reuse the existing parse_pair validation and PauliString::commutes
• add black-box CLI tests for commuting, anticommuting, length-mismatch, and invalid-letter cases

Why

The engine already knows whether two equal-length Pauli strings commute, but
that operation was not available from the command line. This exposes the
existing capability without changing the library API or duplicating
validation logic.

User impact

Users can now run:

qsym commute X Z     # anticommute
qsym commute XX ZZ   # commute

Invalid Pauli letters and unequal lengths continue to use the CLI's existing
nonzero error: <message> path.

Validation

• conda run -n qsym-rs make check
• formatting and clippy passed with warnings denied
• 15 CLI tests passed
• 6 engine tests passed
• documentation tests passed

Closes #1

[hzgpmc]

Self-review record

What was checked

• Issue and design compliance: qsym commute <A> <B> uses the existing
  parse_pair validation and PauliString::commutes, prints exactly
  commute or anticommute, and does not change src/lib.rs.
• Scope: the production change is limited to src/main.rs; the only other
  code change is black-box CLI coverage in tests/cli.rs.
• Success cases: commute X Z reports anticommute, while
  commute XX ZZ reports commute.
• Failure cases: unequal lengths and invalid Pauli letters exit nonzero,
  use the existing error: <message> path, and print no success token.
• Regression: mul XZ ZX remains +YY.
• TDD evidence: all four new tests failed first because commute was an
  unknown subcommand, then passed after the minimal implementation.
• Quality: a task-scoped review and a separate whole-branch review found
  no Critical, Important, or Minor issues.

What was fixed

No review fixes were required; both independent reviews approved the original
implementation.

Verification

conda run -n qsym-rs make check passed:

• formatting check
• clippy with warnings denied
• 15 CLI tests
• 6 engine tests
• documentation tests