[P1] Extend entity_linker to process calendar event attendees

[tinkermonkey]

Problem

Calendar events include attendees: [{"name": str, "email": str}] in domain_metadata. The entity linker (core/entity_linker.py) queries $.sender and $.recipients when building cross-links, but does not query $.attendees[*].email.

A meeting with five contacts generates zero entity_links rows. Queries like "who is attending tomorrow's standup and what's my recent message history with them?" are unanswerable.

Proposed fix

Extend the domain→JSON-path mapping in entity_linker.py to include the events domain attendees:

DOMAIN_EMAIL_PATHS = {
    "messages": ["$.sender", "$.recipients[*]"],
    "events": ["$.attendees[*].email"],          # add this
    "tasks": ["$.collaborators[*]"],             # if applicable
}

The attendee email is already present and normalized in domain_metadata — this is purely an additive query in the linker with no upstream changes required.

Expected outcome

• Every calendar event chunk with a known attendee gets a person_appearance link in entity_links
• "Context about [contact] before [meeting]" becomes a satisfiable query once the entity_links query API is exposed (see P0 issue)

Notes

• Attendee emails should be normalized to lowercase before comparison (contacts emails are already normalized via PeopleMetadata)
• Linker should handle attendees being absent or an empty list gracefully (it may already do so for other optional fields)