[Sync] identityKeys (plural) — multiple cross-platform identity keys per record

[moekatib]

Problem

Today identityKey (singular) extracts one dot-path from a record and writes it as a prefixed key on the mem row (e.g. email:jane@acme.com). That works for 1:1 records like attio/attioPeople — but it doesn't fit records with N participants.

Concrete failure mode: a Gmail thread has many participants (from + to + cc across all messages). There's no single email to pick. A single identityKey is either wrong (picks one arbitrarily) or unset (no cross-platform linking at all — current state).

The same constraint applies to:

• google-calendar/events → each attendee
• Slack messages/threads → each participant
• Fathom meetings → each calendar invitee
• Linear issues → assignee + subscribers

Proposed

Add identityKeys: Array<{ prefix: string, path: string }> alongside identityKey. Each entry resolves via the existing dot-path walker; [] wildcard expands to one key per array element.

{
  "platform": "gmail",
  "model": "threads",
  "identityKeys": [
    { "prefix": "email", "path": "messages[].payload.headers[?(@.name=='From')].value" },
    { "prefix": "email", "path": "messages[].payload.headers[?(@.name=='To')].value" },
    { "prefix": "email", "path": "messages[].payload.headers[?(@.name=='Cc')].value" }
  ]
}

Same lowercase/trim/dedupe behavior as the current identityKey. Each resolved value becomes one entry in the record's keys array.

Acceptance

• identityKeys array accepted in sync profiles alongside (or instead of) identityKey.
• Wildcard paths expand to multiple keys; duplicates deduped.
• Existing identityKey singular keeps working (backwards compatible).
• sync test shows resolved identity keys in its output (how many keys per record, sample values).
• Docs updated in one guide sync under "Cross-Platform Identity".

Related

Enables #129 (gmail), #130 (gcal/fathom), #131 (mem linked query). Found during PR #125 testing — see thread comment.