feat: add Node.js and npm doctor diagnostics

[dDevAhmed]

Summary

Implements CLI-017 by adding Node.js and npm environment diagnostics to the default stellarforge doctor run.

Included

• dependency-free numeric version parser/comparator
• Node.js diagnostic using process.versions.node
• npm availability/version diagnostic using a safe injectable subprocess adapter
• support policy aligned to package engines:
  • Node.js >=22.13.0 <25
  • npm >=10.9.0
• actionable remediation for missing, malformed, and unsupported versions
• default registration of Node/npm checks in doctor
• unit tests for supported/unsupported/malformed/missing cases
• secret-leakage regression test for subprocess failures
• built-CLI integration test proving both default checks pass in the supported CI toolchain
• minor Changeset for the new user-visible capability

Security

• npm is executed as spawnSync('npm', ['--version'])
• shell: false
• no command interpolation
• no environment dump
• stderr/error details from a failed npm process are not rendered to users
• no new runtime or development dependencies

Scope boundaries

This PR does not add Git, Rust/Cargo, Stellar CLI, or Docker diagnostics; those remain CLI-018 through CLI-021.

Closes #20