refactor: review & simplify CLI entry-points (app, runtime, login)

[thomaschristory]

Scope

The primary CLI entry-points and session runtime.

Goal

Code review + simplify pass:

• nsc/cli/app.py (303 lines) — Typer app definition and top-level callbacks
• nsc/cli/runtime.py (320 lines) — per-invocation runtime context
• nsc/cli/login_commands.py (310 lines) — nsc login command group

Check for:

• Global state managed in app.py that should move to runtime.py
• Overly wide callback signatures
• Login flow complexity (credential storage, token refresh, profile creation)
• Opportunities to push output formatting down to output/

Files

• nsc/cli/app.py, nsc/cli/runtime.py, nsc/cli/login_commands.py
• Related tests: tests/cli/

[thomaschristory]

Claiming #51. Branch: refactor/cli-simplify. Reviewing nsc/cli/{app,commands,display,params,state}.py for dead branches, over-defensive conditionals, Any escapes, what-comments.

[thomaschristory]

Done. PR: #68