feat(check): add global speed checker command

[thibaultleouay]

Adds openstatus check <URL> (alias c): a top-level, unauthenticated command that runs a one-shot HTTP check from 28 global regions via the public OpenStatus speed checker.

Highlights:

• Live row-per-region output streaming as NDJSON arrives, followed by a summary footer (fastest/slowest/mean/success-rate + share URL).
• --timing adds DNS / Connection / TLS / TTFB / Transfer columns.
• --json buffers a single object with full timing nested per region, ready for jq pipelines.
• curl-style flags -X / -H / -d (with @file and @- stdin support).
• Typed errors for 429 (parsed Retry-After + body reset fallback), 4xx (with VPN/proxy hint when body mentions client IP), 5xx, network errors, and truncated streams.
• Region codes mapped to human-readable names (e.g. koyeb_tyo → "Tokyo (Koyeb)") from the upstream skill repo snapshot.

Internals:

• New internal/check/ package: client.go (NDJSON streaming parser), render.go (table + summary + JSON output), regions.go (display names), types.go (typed errors).
• internal/api/client.go gains PlayCheckerURL pinned to www.openstatus.dev (skips a 308 redirect on every call).
• Tests use the existing RoundTripper-interceptor pattern from internal/run/run_test.go; fixtures captured from live probes live in internal/check/testdata/.

Docs:

• README: new check row, dedicated Quick Start example, Global Speed Check section.
• Manpage and generated markdown regenerated.
• skills/cli/SKILL.md teaches when to reach for check vs. saved monitors.

Version bumped to v1.2.0 (new top-level command = semver minor).