Add signup command

[usiegl00]

Summary

Adds bld signup -e user@example.com, a fully CLI-driven signup so an agent can sign a user up on their behalf with just an email address.

• Calls Build::DefaultApi#signup(email), which sends a verification link to that address
• No auto-login: the user clicks the link to finish creating the account, then runs bld login to authenticate
• Output notes that region access is granted separately, so a new account may not have access to any region yet
• Signup is unauthenticated, so the command builds Build::DefaultApi directly rather than going through Base#api (which exits when no token is stored)
• On Build::ApiError, shows the response body's error JSON field (falling back to the raw message) and exits non-zero
• Registers the command in build_cli.cr and documents it in bld skills per AGENTS.md

⚠️ Does not compile yet — SDK dependency

DefaultApi#signup does not exist in the generated SDK yet. It is added by a follow-up buildio/sdk-crystal-lang PR that should reference this PR. Until that SDK update is pinned here, this branch fails to build — expected and intentional per the agreed workflow (CLI side first, SDK/server side after).

Expected generated method (mirroring oidc_login): DefaultApi#signup(email : String? = nil) — raises Build::ApiError on non-2xx, where the error body may contain a JSON "error" message for display.

Test plan

• bld signup --help and missing --email error path verified against a locally built binary (prior raw-HTTP iteration; command wiring unchanged)
• shards build / crystal spec once the SDK adds signup
• End-to-end verification against the real endpoint

🤖 Generated with Claude Code