[dDevAhmed]

Summary

Implements CLI-016 by adding the stellarforge doctor command shell and an internal diagnostic aggregation model without implementing the individual environment checks yet.

Included

• typed diagnostic status/result/check/report model
• modular internal diagnostic aggregation
• safe conversion of unexpected check exceptions into generic failed diagnostics
• deterministic pass/warn/fail rendering through the existing terminal output layer
• stellarforge doctor registration in the root command
• stable diagnostic exit code 5 for completed health checks with failed required diagnostics
• exit-code reference update
• tests for empty diagnostics, mixed pass/warn/fail aggregation, remediation rendering, safe exception handling, and exit behavior

Scope boundaries

This PR does not implement Node/npm, Git, Rust/Cargo, Stellar CLI, or Docker checks. Those remain CLI-017 through CLI-021.

It also does not introduce a public plugin system; checks are modular only as an internal implementation boundary.

Security

• no shell execution
• no environment or secret dumping
• unexpected diagnostic exceptions do not expose original error messages
• output continues through the existing terminal sanitizer
• no new dependencies

Exit semantics

• no failed diagnostics: exit 0
• one or more failed diagnostics: exit 5
• warnings alone remain non-fatal

Closes #19