test: add coverage for email identity fallback chain

[MathurAditya724]

Tracking issue from review of #13.

identifyEmail in packages/opencode-webhooks/src/email/identity.ts walks a fallback chain (Message-ID → In-Reply-To → References[i]) but has no tests. A regression in regex order or references iteration order would only surface at runtime.

The repo has no test infrastructure today; this issue tracks adding it alongside coverage for at least:

• identification via message_id (primary path)
• fallback to in_reply_to when message_id is unrecognized
• fallback to a matching entry in references[] when both above miss
• all-unknown input returns { kind: "unknown" }

Once test infra exists, these should be straightforward unit tests against the pure identifyEmail function.