feat: add Stellar CLI environment diagnostic

[dDevAhmed]

Summary

Implements CLI-020 by adding a safe doctor diagnostic for the current official Stellar CLI.

Included

• invokes only stellar version --only-version for machine-friendly version detection
• does not inspect identities, wallet keys, seed phrases, network aliases, or unrelated Stellar configuration
• validates a documented compatibility floor of Stellar CLI 27.1.0, matching the current recommended Protocol 27 Mainnet software matrix
• accepts normalized semantic versions, including a leading v
• returns actionable missing, malformed, and incompatible-tool remediation
• integrates the Stellar diagnostic into the default stellarforge doctor check set
• adds unit coverage for command arguments, supported/newer versions, old versions, missing CLI, malformed output, and secret-safe failures

Security

• no shell interpolation
• no wallet/config inspection
• no network request in the diagnostic
• subprocess stderr/errors are not rendered into diagnostic output
• uses the existing injectable RunCommand boundary

Reference

Stellar's official CLI manual documents stellar version --only-version. Stellar's current software-version matrix lists CLI 27.1.0 for Protocol 27 Mainnet.

Closes #23