feat(cli): Add basic CLI with run command

[TobiasGleiter]

Summary

Add a minimal CLI so the agent can be used against a local Go codebase.

Command

gogantic run ""

Example

gogantic run "refactor the authentication handler to use the new middleware"

Requirements

• Single binary
• run command accepts a task as a string argument
• Reads target directory from flag, defaults to current directory
• Prints ReAct steps to stdout as the agent runs
• Exits with non-zero code on failure

Out of Scope

• Auth
• Config files
• Web UI
• Audit command (separate issue)