[cli] Update codebase to argmojo v0.5.0 and use declarative API

[forfudan]

This PR updates the CLI calculator to use ArgMojo v0.5.0 and migrates the argument parsing code to ArgMojo’s declarative Parsable API (hybrid, with builder API).

Changes:

• Replaced imperative Command/Arg setup with a DecimoArgs(Parsable) definition and parsing via to_command() / parse_from_command().
• Switched the project to depend on argmojo >=0.5.0,<0.6.0 and simplified CLI build steps to rely on the packaged dependency.
• Removed the CI “fetch argmojo” step now that ArgMojo is pulled in via pixi.

[Copilot]

Pull request overview

Updates the CLI calculator to use ArgMojo v0.5.0 and migrates the argument parsing code to ArgMojo’s declarative Parsable API.

Changes:

• Replaced imperative Command/Arg setup with a DecimoArgs(Parsable) definition and parsing via to_command() / parse_from_command().
• Switched the project to depend on argmojo >=0.5.0,<0.6.0 and simplified CLI build steps to rely on the packaged dependency.
• Removed the CI “fetch argmojo” step now that ArgMojo is pulled in via pixi.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/cli/main.mojo	Migrates CLI arg parsing to ArgMojo’s declarative Parsable model and adds help tips.
pixi.toml	Adds ArgMojo v0.5 dependency and updates tasks/build steps to stop using a local clone by default.
pixi.lock	Locks ArgMojo v0.5.0 for supported platforms.
.github/workflows/run_tests.yaml	Removes the step that cloned ArgMojo in CI.

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