feat: return contact relations and omit binary vCard payloads - #4

get-contact echoed the stored vCard verbatim. A card holding an inline
portrait carries PHOTO;ENCODING=b with tens of thousands of base64
characters; on one real card that was 57 KB of a 60 KB result. For an MCP
client this is context spent on bytes no model can use, and it can push a
single tool result past the response limit. Embedded binary values (PHOTO,
LOGO, SOUND, KEY) are now reported as a size marker instead. A photo
referenced by URI is untouched.

Relations are surfaced as a `relations` object keyed by role, e.g.
{"Spouse": "Ada Lovelace"}. Clients record them as a pair sharing a property
group - itemN.X-ABLABEL carries the role, itemN.X-ABRELATEDNAMES the person,
with the role wrapped in Apple's _$!<...>!$_ form - which every caller would
otherwise have to reassemble from raw vCard text.

`relations` mapped a role to a single name, so a card recording two children
under the same label kept only the last one and dropped the other without any
signal. Children, siblings and friends are exactly the roles that repeat, so
this is the ordinary case rather than a contrived one. Each role now carries a
list, and a person reached twice — as X-SPOUSE and an item pair usually do —
is recorded once.

Also spells out in the tool description that the returned vCard is normalised
rather than byte-for-byte, so nobody mistakes it for the stored copy.