feat: move a chat family between projects in chatd - #30893

chatd can now move a root chat, together with its subagent chats, into a project, to another project, or out of its project. Nothing calls this yet. #30894 adds the API.

Stack (moving chats between projects, bottom first):

1. This PR: the move in chatd
2. feat: move chats between projects through the chat update API #30894: PATCH /api/v2/chats/{chat} takes project_id
3. feat(site/src): move chats between projects from the chat menu #30895: Move to project in the chat menu
4. feat(site/src/pages/AgentsPage/components): drag chats onto project folders in the sidebar #30795: drag a chat onto a project folder in the sidebar

Why: today a chat keeps the project it was created with. A move must follow the family rules of archiving, because a step that runs during a move would still have the memory tools of the old project.

• chatstate.Tx.SetProject runs only from W, E0 and E1, like SetArchived. SetFamilyProject() locks the root row before its subagents, in the same order as archiving, so a busy family gets ErrTransitionNotAllowed and nothing changes.
• A move to the project the chat is already in changes nothing, even while the chat runs. The check reads the locked root row, so it waits for a move that is still in flight.
• Only the root row stores project_id, and subagents use the project of the root. After the commit, chatd publishes a new project_change watch event for each chat of the family. The move keeps updated_at, so lists keep their order.
• A root chat outside a project now has an empty memory index. When a chat leaves its project, or its project is deleted, its next turn start commits one memory index update that removes the memories the model saw there.

What to scrutinize: the lock order and the same-project check under the lock in SetFamilyProject() (chatstate/family.go), and the turn-start condition in memoryIndexMessage() (memory.go).

Generated (make gen): queries.sql.go, querier.go, dbmock and dbmetrics come from queries/chats.sql. coderd/apidoc, docs/reference/api/schemas.md and typesGenerated.ts come from the new watch event kind, which is also why two chats.test.ts fixtures gain project_change: false.

coderd/x/chatd/ARCHITECTURE.md has no edit. Lines 60 (the project of a root chat can now change), 126 (the transition list needs SetProject), 470 (the move follows the family rules of archiving) and 1009 (a chat that leaves its project gets its memory index removed) need the author's update.

Testing

• make pre-commit passes. go test passes for coderd/x/chatd/chatstate, coderd/database/dbauthz (TestMethodTestSuite) and the chatd memory tests.
• TestSetFamilyProjectLocksRootFirst fails with 55P03 when a subagent row is locked first. TestSetFamilyProjectKeepsTheCurrentProject covers the same-project move during a run. The transition matrix test gains SetProject cases for W, E0 and E1.
• TestMovedChatUsesNewProject runs two real turns around a move: the second turn uses the instructions and memories of the new project, and OutOfProject fails without the memory index removal.

Xum opened this PR on behalf of @ibetitsmike.