emulate adds Linear API sandbox

This is niche infrastructure, but it hits a real pain point for AI agent builders: testing workflow integrations without live SaaS accounts, rate limits, or flaky webhooks.

• –Linear is becoming a common control plane for coding agents, so local issue/comment/project emulation makes agent test suites more realistic.
• –GraphQL mutations updating in-memory state are more useful than static mocks because SDK behavior, pagination, auth, and webhook delivery can be exercised together.
• –Signed Linear-shaped webhooks matter for CI because teams can test verification and retry paths without exposing tunnels or touching production secrets.
• –The limits are clear: this is not full Linear parity, so teams still need contract tests against the real API for edge cases.