Link Chat To Trip

File a chat onto a trip (link_source='item_save') — the user's own tag.

Drives "save a card out of a tripless chat": the user picks a destination trip, this files the chat there, and the item save itself then goes through the normal itinerary route.

Filing, not binding (docs/GLOBAL_CHATS.md §4.2). This is the one path that can give a global chat a primary link, and it is deliberately the only one: it is the user saying where the chat belongs. It does NOT change which trip the chat's agent reads or writes — that stays the birth binding (None for a global chat), so tagging a chat never rewrites its tool list or its addressing scheme mid-conversation.

Idempotent: a chat that already has a primary link gets it back untouched rather than being relinked — which is also what makes a double-tap safe.

"Unlinked" here means no primary, not no links: a global chat can hold N reference links and none primary, and saving into a trip it already references promotes that link rather than minting a second one.