refactor: prune session-store and agent-list dead code

The session_entries supersede pipeline was a design that never
shipped: `SqliteSessionStore.replaceMessages` / `appendCompaction`,
`sessionEntries.replace` / `listAll` / `deleteForSession`, and the
`"compaction" | "summary"` kind variants had zero live callers.
Compaction wires through pi's `transformContext` and leaves the
persisted transcript alone, so the shadow-row machinery was pure
cruft with two tests propping it up.

Rip it out: the store exposes only `loadMessages` and
`appendMessage`, `sessionEntries` exposes only `append` and
`listActive`, and `SessionEntryKind` narrows to `"message"` so new
writers can't re-introduce a dead kind by accident.
`is_superseded` stays on the table — the DB column is harmless
with no writer, and dropping it is a separate schema migration
concern.

`listTopLevelAgentIds` loses its only caller in the previous
commit (company channel now fans out through
`listCompanyBroadcastAgentIds`), so it goes too. Three tests that
validated the removed behaviour are deleted; the one remaining
session-store test still covers the live `append + load` path.

src/main/agent-runtime/sqlite-session-store.ts

@@ -4,9 +4,12 @@ import type { AppDatabase } from "@main/database";
 /**
  * SQLite-backed pi `AgentMessage[]` store.
  *
- * One store instance per task_session. Owners must call `appendMessage`
- * exactly once per pi `message_end` event, and `replaceMessages` after pi
- * `compact` returns a compacted message list.
+ * One store instance per task_session. `appendMessage` runs exactly
+ * once per pi `message_end` event; `loadMessages` reads the live
+ * transcript back at `prepareRun`. Compaction happens at LLM-call
+ * time via pi's `transformContext` hook and does not rewrite the
+ * persisted transcript — the session shadow is the durable truth,
+ * the compacted payload is a per-call view.
  */
 export class SqliteSessionStore {
   constructor(
@@ -33,28 +36,4 @@ export class SqliteSessionStore {
       payloadJson: JSON.stringify(message),
     });
   }
-
-  appendCompaction(summary: string): void {
-    this.db.sessionEntries.append({
-      taskSessionId: this.taskSessionId,
-      kind: "compaction",
-      role: null,
-      payloadJson: JSON.stringify({ summary }),
-    });
-  }
-
-  replaceMessages(messages: AgentMessage[], compactionSummary?: string): void {
-    const entries: Array<{ kind: "message" | "compaction"; role: string | null; payloadJson: string }> = [];
-    if (compactionSummary) {
-      entries.push({ kind: "compaction", role: null, payloadJson: JSON.stringify({ summary: compactionSummary }) });
-    }
-    for (const message of messages) {
-      entries.push({
-        kind: "message",
-        role: (message as { role?: string }).role ?? null,
-        payloadJson: JSON.stringify(message),
-      });
-    }
-    this.db.sessionEntries.replace({ taskSessionId: this.taskSessionId, entries });
-  }
 }