feat(cli): add weave doctor — verify MCP servers are reachable

[breferrari]

Summary

Add a weave doctor command that verifies installed MCP servers are actually functional, beyond just checking config correctness.

Motivation

weave diagnose checks that config files are consistent with installed packs. But it doesn't verify that the configured MCP servers actually work. A user can have a perfectly consistent config that references a server binary that isn't installed or an HTTP endpoint that's unreachable.

Suggested checks

• stdio servers: verify that the command exists in PATH (e.g., npx, uvx, custom binaries)
• HTTP servers: verify the url is reachable (HEAD request with timeout)
• Package availability: for npx/uvx servers, optionally check that the package exists in the registry
• Environment variables: check that required env vars declared in the pack are set

Output

Report per-server status (ok/warn/error) with actionable messages for failures.