[mojo][core] Update the codebase to Mojo v1.0.0b2

[forfudan]

This PR updates ArgMojo’s core code and tooling configuration to be compatible with Mojo v1.0.0b2.

[Copilot]

Pull request overview

This PR updates ArgMojo’s codebase and development tooling to target Mojo v1.0.0b2, including adjustments to reflection usage and environment/build configuration.

Changes:

• Update reflection call sites to the Mojo v1.0.0b2 reflect[...] form and remove now-unneeded reflection imports.
• Bump project/tooling versions (ArgMojo v0.7.0, Mojo v1.0.0b2) and update Pixi tasks/lockfile accordingly.
• Minor docstring/doc updates (tests headers, added Raises: sections, changelog entry).

Reviewed changes

Copilot reviewed 9 out of 11 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/test_options.mojo	Docstring header formatting tweak.
tests/test_groups.mojo	Docstring header formatting tweak.
tests/test_completion.mojo	Docstring header formatting tweak.
src/argmojo/parsable.mojo	Update reflection usage; expand docstrings with Raises:.
src/argmojo/command.mojo	Document parse() error/raise behavior.
src/argmojo/argument_wrappers.mojo	Update reflection usage; remove unused import; expand docstrings with Raises:.
README.md	Update stated ArgMojo/Mojo target versions.
pixi.toml	Bump version + Mojo dependency; change package task output/artifact; update clean task.
pixi.lock	Refresh locked dependencies for updated environment.
docs/changelog.md	Add v0.7.0 entry for Mojo v1.0.0b2 migration.
.gitignore	Update ignored build artifact extensions to .mojoc.

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