[Sync] Built-in gmail/threads profile: identity keys for all participants

[moekatib]

Problem

The current `gmail/threads` sync profile has no `identityKey` declared. Even once #128 lands (plural identityKeys), the built-in profile needs to actually use it so Gmail threads join to Attio/HubSpot contacts via shared email.

Today the profile uses a jq transform that only extracts `from_email` (first sender of first message) as a flat column — not as an identity key, and it misses to/cc/bcc participants.

Proposed

Update the built-in `gmail/threads` profile to:

1. Declare `identityKeys` (from [Sync] identityKeys (plural) — multiple cross-platform identity keys per record #128) covering every message's `From`, `To`, `Cc` headers across the whole thread.
2. Normalize values: strip display names (`"Jane jane@acme.com"` → `jane@acme.com`), lowercase, trim.
3. Move the existing `from_email` extraction out of the user-editable `gmail-extract.jq` and into the built-in profile so users get this by default without copying a transform.

Acceptance

• Fresh `sync init gmail threads` produces a profile with participant identity keys populated.
• Running `sync run gmail --models threads` writes records with keys like:
  `["gmail/threads:", "email:moe@withone.ai", "email:anish@intently.ai", ...]`
• Query `one mem list gmail/threads` shows N identity keys per thread (not 0, not 1).
• Built-in profile's `sync test` preview shows the resolved identity keys.

Blocked on

#128 (identityKeys plural)

Related

Part of the cross-platform identity story. Found during PR #125 testing.