refactor(core): simplify prompt inbox promotion and projection

Remove the steer promotion cutoff: a turn boundary now promotes every
steer pending at promotion time instead of snapshotting the inbox at a
captured sequence, so late-arriving steers ride along into the turn.

Delete legacy projected-prompt synthesis from projectPrompted. The
event-sourced session input migration wiped pre-inbox event history,
so every Prompted event follows an admitted inbox row and a missing or
divergent row on replay is an invariant violation.

Inline the prompt equivalence predicate and update tests that seeded
history with bare Prompted events to publish PromptAdmitted first.

packages/core/test/session-compact.test.ts

@@ -81,11 +81,20 @@ describe("SessionV2.compact", () => {
       const events = yield* EventV2.Service
       const created = yield* session.create({ location })
 
+      const messageID = SessionMessage.ID.create()
+      const prompt = Prompt.make({ text: "Please compact this session history." })
+      yield* events.publish(SessionEvent.PromptAdmitted, {
+        sessionID: created.id,
+        messageID,
+        timestamp: DateTime.makeUnsafe(0),
+        prompt,
+        delivery: "steer",
+      })
       yield* events.publish(SessionEvent.Prompted, {
         sessionID: created.id,
-        messageID: SessionMessage.ID.create(),
+        messageID,
         timestamp: DateTime.makeUnsafe(0),
-        prompt: Prompt.make({ text: "Please compact this session history." }),
+        prompt,
         delivery: "steer",
       })