Copy docs/design/UNIFIED_CHAT_UX.md verbatim from PR #138 as the in-tree
UX ceiling. Body unedited per user directive; a prepended FE-716 reading
note documents vocabulary (thread = secondary chat) and substrate (D153
defers a schema-level thread table; FE-716 uses chat columns + existing
chat.kind).

Co-authored-by: Amp <amp@ampcode.com>

Add nullable parent_chat_id, invoked_in_turn_id, pinned_item_id,
pinned_span_hint to the chat table plus indexes on parent_chat_id and
invoked_in_turn_id (migration 0020). Zero enum changes — chat.kind stays
['interview', 'side_chat']. Tests cover column shape, index presence,
FK rejection, and active_turn_id preservation.

Co-authored-by: Amp <amp@ampcode.com>

createSecondaryChat inserts a chat row with kind='side_chat' and the
four C1 columns. createKickoffTurn inserts a turn with
turn_kind='kickoff' and resolves specification_id from the chat. POST
route deferred to C3 (no client consumer yet). Tests cover happy paths,
optional columns, and FK rejection.

Co-authored-by: Amp <amp@ampcode.com>

listSecondaryChatsForSpecification returns chat rows where
parent_chat_id IS NOT NULL with their earliest kickoff turn.
readSpecificationStateProjection includes a new secondaryChats field;
secondaryChatStateSchema added to api-types.ts (additive). Tests cover
empty, single-with-kickoff, missing-kickoff, primary-chat exclusion,
multi-spec scoping, and bundle round-trip.

Co-authored-by: Amp <amp@ampcode.com>

Radix Collapsible-backed surface keyed off the secondaryChats wire
shape. Header always renders; body collapsed by default and reveals the
kickoff turn's assistant_parts on expand; tolerates kickoffTurn=null.
Mounting in the workspace view is deferred to C3c-mount. Tests cover
header render, default collapsed, expand on click, and empty body
fallback.

Co-authored-by: Amp <amp@ampcode.com>

handleCreateSecondaryChatRequest validates spec + body (parentChatId,
invokedInTurnId, itemKind, itemId, spanHint?), resolves the item via
getKnowledgeItem (rejects when missing or kind/spec mismatch), then
calls createSecondaryChat + createKickoffTurn and returns {chatId,
kickoffTurnId}. Existing POST /side-chat untouched (deletion is C8).
Per-mode kickoff templates land in C4. Tests cover happy path with
bundle round-trip, span-hint persistence, 400 on bad body, 404 on
missing spec, 404 on missing item.

Co-authored-by: Amp <amp@ampcode.com>

…turn

Project specificationState.secondaryChats into a
secondaryChatsByInvokedTurnId map on ContinuousWorkspaceController and
thread it through ContinuousWorkspaceView into
WorkspaceTranscriptArtifacts. getArtifactAnchorTurnId covers all
turn-bearing artifact kinds. SecondaryChatCollapsible renders inline
under each matching anchor in a secondary-chats-for-turn-{id} slot;
chats without a matching artifact never render (no-orphan invariant).

Co-authored-by: Amp <amp@ampcode.com>

…button

SecondaryChatTriggerProvider derives parentChatId (= primary_chat_id)
and invokedInTurnId (= active_turn_id) from the spec bundle and exposes
create({kind, id}). useCreateSecondaryChatMutation POSTs to
/api/specifications/:id/secondary-chats and invalidates the bundle.
Mount the provider in the specification route. Add an 'Open inline
chat' button on the structured list-view ItemActionRail.
specificationSchema gains primary_chat_id?: number for transition.
Mode wiring follows in C4.

Co-authored-by: Amp <amp@ampcode.com>

Add nullable chat.mode column ('explore' | 'edit') via migration 0021.
createSecondaryChat defaults to 'explore'; setSecondaryChatMode rejects
non-secondary chats. Project mode through the bundle. Expose via PATCH
/api/specifications/:id/secondary-chats/:chatId/mode. Client adds
useSetSecondaryChatModeMutation; SecondaryChatCollapsible gets an
Ask/Edit toggle (sibling of CollapsibleTrigger to avoid nested
buttons); WorkspaceTranscriptArtifacts wires it via a thin
SecondaryChatCollapsibleWithMode wrapper. Streaming-with-tools wiring
deferred to C5. Update CARDS.md to mark C3c-mount, C3c-wire, C4 done.

Co-authored-by: Amp <amp@ampcode.com>