Implement Git-mastery REPL

[jiaxinnns]

RFC: https://docs.google.com/document/d/1Xr-ppB1TLQtQB_tvPvOtSi2_h4xAhqMEEM4JxbVy368
Fixes git-mastery/git-mastery#68

[Copilot]

Pull request overview

This PR implements an interactive REPL (Read-Eval-Print Loop) for Git-Mastery, allowing users to execute Git-Mastery commands and shell commands in an interactive session. The REPL provides a convenient way to work with Git-Mastery without repeatedly typing the full command prefix. Additionally, it updates the module caching mechanism in the exercise loader to ensure fresh imports in the REPL context.

Changes:

• Added a new interactive REPL command with support for Git-Mastery commands, shell command pass-through, and built-in commands like cd, exit, and help
• Enhanced module cleanup in the exercise loader to prevent stale module caches in persistent REPL sessions
• Integrated the new REPL command into the CLI command registry

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
app/commands/repl.py	Implements the interactive REPL with command execution, shell pass-through, and help system
app/utils/gitmastery.py	Adds module cache cleanup before and after exercise file execution to ensure fresh imports in REPL context
app/commands/init.py	Exports the new repl command for use in the CLI
app/cli.py	Registers the repl command in the CLI command list

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