fix(server): apply get_agent_activity limit to projected entries

The MCP get_agent_activity tool sliced the raw timeline before passing
to the curator, so `limit: 5` could land mid-message and return a
trailing fragment of streamed assistant text. Now goes through the
existing projection pipeline so the limit counts whole projected
entries — assistant deltas merged, tool calls collapsed by callId,
reasoning chunks merged.

Same bug fixed in the wait_for_agent timeout's "Recent activity"
preview, which used the same raw slice.

Consolidates the merge logic: activity-curator no longer ships its own
collapseTimeline, it delegates to projectTimelineRows. One source of
truth for "what is a complete message".

Drops the meaningless `provider` parameter that was threaded through
the projection module — merge logic never branched on it. The wire
schema keeps `provider` per entry; session stamps it once at the emit
boundary. Adds `reasoning_merge` to the projection kind enum (additive
wire change).

packages/server/src/shared/messages.ts

@@ -2390,7 +2390,7 @@ export const AgentTimelineEntryPayloadSchema = z.object({
   seqStart: z.number().int().nonnegative(),
   seqEnd: z.number().int().nonnegative(),
   sourceSeqRanges: z.array(AgentTimelineSeqRangeSchema),
-  collapsed: z.array(z.enum(["assistant_merge", "tool_lifecycle"])),
+  collapsed: z.array(z.enum(["assistant_merge", "reasoning_merge", "tool_lifecycle"])),
 });
 
 export const FetchAgentTimelineResponseMessageSchema = z.object({