Add assignment email digest support

[victorhblancom-design]

Fixes #6.

Summary

• add optional assignee email capture and persist sanitized assignee emails with tasks
• queue assignment changes into delayed hourly digest batches instead of sending immediately
• add a real unsubscribe endpoint backed by stable per-recipient tokens
• add Resend delivery support plus a local log provider for development
• document the email provider environment variables

Why this approach

The digest queue is stored with TaskForge data, so pending notifications survive process restarts. New assignments are detected by comparing the previous task list to the submitted one; repeated saves of the same assignee do not enqueue duplicate emails. Unsubscribe links clear pending work and prevent future digests for that recipient.

Verification

• C:\Users\Vick_\.cache\codex-runtimes\codex-primary-runtime\dependencies\node\bin\node.exe --test

All 7 tests pass.

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[victorhblancom-design]

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