feat: add Git doctor diagnostic

[dDevAhmed]

Summary

Implements CLI-018 by adding Git availability/version detection to the default stellarforge doctor run.

Included

• Git diagnostic using the existing safe injectable subprocess adapter
• git --version execution with an argument array and shell: false
• parsing for standard and platform-suffixed Git version output
• clear failure/remediation when Git is missing, cannot execute, or returns malformed version output
• default doctor registration after Node.js/npm checks
• unit tests for success, platform suffixes, missing Git, malformed output, and secret-leak prevention
• built-CLI integration coverage updated to expect Node.js, npm, and Git
• minor Changeset for the new user-visible diagnostic

Security

• no shell interpolation
• no repository, identity, credential, remote, or Git config inspection
• subprocess stderr/error details are not surfaced to users
• no new dependencies

Scope boundaries

This issue checks only Git availability/version. Rust/Cargo, Stellar CLI, and Docker remain CLI-019 through CLI-021.

Closes #21