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/database-base.ts

@@ -252,7 +252,6 @@ export class AppDatabaseBase {
   setHeartbeatPolicy(agentId: string, companyId: string, policy: { enabled?: boolean; intervalSec?: number; cooldownSec?: number; maxConcurrentRuns?: number; wakeOnDemand?: boolean }) { return this.agents.setHeartbeatPolicy(agentId, companyId, policy); }
   countActiveRunsForAgent(agentId: string, companyId: string) { return this.agents.countActiveRunsForAgent(agentId, companyId); }
   listDepartmentAgentIds(companyId: string, department: string) { return this.agents.listDepartmentAgentIds(companyId, department); }
-  listTopLevelAgentIds(companyId: string) { return this.agents.listTopLevelAgentIds(companyId); }
   listCompanyBroadcastAgentIds(companyId: string) { return this.agents.listCompanyBroadcastAgentIds(companyId); }
   listIncidentPingableAgentIds(companyId: string) { return this.agents.listIncidentPingableAgentIds(companyId); }
   findAgentName(agentId: string) { return this.agents.findAgentName(agentId); }