CLI: Add `w3-kit doctor` command for environment diagnostics

[PetarStoev02]

Context

When users hit issues with the CLI, the first question is usually "what does your env look like?" — node version, network access, registry reachable, package versions, etc. A doctor command surfaces all of this in one place and gives users something concrete to paste into bug reports.

What to build

Implement w3-kit doctor that runs a battery of checks and prints a readable report:

• Node version (and whether it meets the supported range)
• npm/pnpm/bun version
• OS / arch
• Network: can the CLI reach the registry source / GitHub / npm?
• Installed @w3-kit/* package versions in the current project (if any)
• Any obvious config problems

Each check should be a single ✓ or ✗ line with an inline explanation when it fails.

Acceptance criteria

• w3-kit doctor runs all checks without crashing even if some fail
• Exit code reflects overall health (0 if all green, non-zero if any check fails)
• Output is concise and copy-pasteable
• --json flag (see related issue) also works here, returning a structured report
• Tests mock the environment so checks are deterministic

Pointers

• Related: cli --json flag

Complexity

🟡 Intermediate