[core] Add auto dispatch + Extend the mgit example

[forfudan]

This PR adds Cobra-style auto-dispatch to ArgMojo commands (register per-command handlers and run via execute()), expands the mgit example to use the new dispatch model (including nested subcommands), and wires in tests + tooling updates to exercise the feature.

Changes:

• Introduce Command.set_run_function(handler) and Command.execute() with recursive subcommand dispatch (plus _execute_with_arguments() test helper).
• Add a dedicated test suite validating dispatch behavior across root/subcommands/nested commands, aliases, persistent flags, and error cases.
• Extend documentation and examples (mgit) to demonstrate and describe auto-dispatch; adjust Pixi tasks to include the new tests and use a new example build script.

[Copilot]

Pull request overview

Adds Cobra-style auto-dispatch to ArgMojo commands (register per-command handlers and run via execute()), expands the mgit example to use the new dispatch model (including nested subcommands), and wires in tests + tooling updates to exercise the feature.

Changes:

• Introduce Command.set_run_function(handler) and Command.execute() with recursive subcommand dispatch (plus _execute_with_arguments() test helper).
• Add a dedicated test suite validating dispatch behavior across root/subcommands/nested commands, aliases, persistent flags, and error cases.
• Extend documentation and examples (mgit) to demonstrate and describe auto-dispatch; adjust Pixi tasks to include the new tests and use a new example build script.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/argmojo/command.mojo	Implements auto-dispatch via stored handler and recursive _dispatch() invoked from execute() / _execute_with_arguments().
tests/test_dispatch.mojo	New tests covering dispatch routing, nesting, handler replacement, aliases, persistent flags, and missing-handler errors.
README.md	Adds auto-dispatch to the feature list.
pixi.toml	Runs the new dispatch tests; replaces the inline build task with examples/build.sh.
examples/mgit.mojo	Refactors/extends mgit to register handlers and use execute(); adds nested subcommands (stash/config/remote).
examples/build.sh	New script to build (and run --help) examples with basic timing output.
docs/user_manual.md	Documents auto-dispatch usage and updates the cross-library method reference tables.
docs/changelog.md	Notes auto-dispatch in the unreleased section.
docs/argmojo_overall_planning.md	Marks auto-dispatch as done and updates planning/status text accordingly.

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[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated 3 comments.

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