One-touch `numan migrate` command for nupm adopters

[tonythethompson]

Problem

#19 gives numan nupm status/inspect a structured, machine-readable eligibility report, but a nupm user still has to run multiple commands by hand (status, inspect, import, activate) to actually move over. That's friction most early adopters won't push through.

Goal

A single numan migrate command that orchestrates the existing nupm compatibility layer end-to-end for everything already proven safe to import, and gives a clear, actionable report for everything that isn't.

Proposed contract

• numan migrate runs nupm status internally, imports every package classified importable_now via the existing nupm import machinery, then runs activate — one atomic user-facing action.
• --dry-run prints exactly what would be imported/activated without mutating state.
• A snapshot is created before any mutation, per the existing "state snapshots before mutation" rule (create_snapshot()).
• Packages outside importable_now (per Nupm migration planner with structured compatibility outcomes #19's reason codes) are reported with their compatibility class and recommended next action — never silently skipped.
• Re-running numan migrate after Numan gains support for a previously-ineligible package should pick it up without manual re-classification steps.

Acceptance criteria

• On a real nupm fixture install, one numan migrate command reaches an activated-equivalent state for the importable_now subset with zero manual config.nu edits.
• Non-importable packages are listed with reason codes and next actions, matching Nupm migration planner with structured compatibility outcomes #19's JSON schema.
• Real-Nu acceptance test covers a mixed fixture (some importable, some not) and asserts no Nu-state mutation occurs for anything outside importable_now.

Non-goals

• Automatically resolving manual_migration_required packages — that stays a human decision.
• Achieving nupm parity — this is a staged bridge, same framing as Nupm migration planner with structured compatibility outcomes #19.