[cli] Enable negative expressions as positional arguments

[forfudan]

This PR updates the Decimo CLI argument parsing so expressions that start with - (e.g., negative numbers / negative expressions) can be passed as the positional <EXPR> more naturally, and documents/tests the behavior.

Changes:

• Enable hyphen-prefixed values for the expr positional argument via ArgMojo (allow_hyphen=True) and remove the older command-level negative-number allowance.
• Expand CLI integration tests to cover negative expressions, mixed option/positional ordering, and -- separator cases.
• Add user-manual documentation for “Negative Expressions” and update the CLI calculator plan checklist accordingly.

[Copilot]

Pull request overview

This PR updates the Decimo CLI argument parsing so expressions that start with - (e.g., negative numbers / negative expressions) can be passed as the positional <EXPR> more naturally, and documents/tests the behavior.

Changes:

• Enable hyphen-prefixed values for the expr positional argument via ArgMojo (allow_hyphen=True) and remove the older command-level negative-number allowance.
• Expand CLI integration tests to cover negative expressions, mixed option/positional ordering, and -- separator cases.
• Add user-manual documentation for “Negative Expressions” and update the CLI calculator plan checklist accordingly.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 6 comments.

File	Description
src/cli/main.mojo	Allows expr positional to start with - and removes allow_negative_numbers() usage.
tests/test_cli.sh	Adds integration coverage for negative expressions and option/positional ordering.
docs/user_manual_cli.md	Documents negative-expression usage (needs clarification for option-like collisions).
docs/plans/cli_calculator.md	Updates planning checklist notes for the new approach (one option name inconsistency).

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