refactor(messaging): project inter-agent messages into pi session transcript

Inter-agent messages previously entered the recipient's context as a
separate prompt block (recentMessages), rebuilt every turn. This double-
charged tokens and kept messages outside pi's native AgentMessage stream.

Now at prepareRun: pull the recipient's unread inbox, project each
AgentMessageRecord into a pi user-role AgentMessage, persist to
SqliteSessionStore, and mark delivered. The recipient reads messages
as natural conversation turns in the transcript.

- db: listAgentInboxUnread + markAgentInboxDelivered (transactional)
- agent-runtime/inbox-projection.ts: AgentMessageRecord -> pi AgentMessage
- runtime-bridge.prepareRun: inject + deliver before loadMessages
- prompt-builder: delete recentMessages context block + buildMessagesSection
- message-normalize: drop unused normalizeMessagesForPrompt
- delete prompt-builder/attachment-resolver.ts (orphan after section removal)

Send path (api-server/routes-messages, ipc, UI) unchanged — inbox table
remains the UI source. Transactional delivery marking prevents duplicate
injection across runs. Failed append -> no mark -> retry on next run.

src/main/database.ts

@@ -19,6 +19,8 @@ export class AppDatabase extends AppDatabaseBase {
   listAgentMessages(companyId: string, opts?: { agentId?: string; messageId?: string; channel?: string; channelTargetId?: string | null; unreadOnly?: boolean; limit?: number }) { return this.messages.listAgentMessages(companyId, opts); }
   searchMessages(companyId: string, query: string, opts?: { channel?: string; channelTargetId?: string | null; limit?: number }) { return this.messages.searchMessages(companyId, query, opts); }
   markMessageReadForReader(messageId: string, companyId: string, readerAgentId?: string | null) { return this.messages.markMessageReadForReader(messageId, companyId, readerAgentId); }
+  listAgentInboxUnread(companyId: string, agentId: string) { return this.messages.listAgentInboxUnread(companyId, agentId); }
+  markAgentInboxDelivered(agentId: string, messageIds: string[]) { return this.messages.markAgentInboxDelivered(agentId, messageIds); }
 
   // ─── Social & Browser Actions ──────────────────────────────────────────────