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/agent-runtime/inbox-projection.ts

@@ -0,0 +1,96 @@
+/**
+ * Inbox projection — converts a stored AgentMessageRecord into a pi
+ * AgentMessage (user role) so inter-agent communication enters the
+ * recipient's transcript as natural conversation rather than a prompt
+ * block re-rendered every turn.
+ *
+ * The projection is the single source of truth for how an inbound message
+ * looks inside a recipient's context: `[Message from {sender}{· channel}]`
+ * header, optional subject line, then the body. Content is normalized
+ * (control characters stripped, oversized bodies truncated) via the same
+ * helpers used by the legacy prompt-builder boundary.
+ */
+
+import type { AgentMessage } from "@mariozechner/pi-agent-core";
+import type { AgentMessageRecord } from "@shared/types";
+
+const MAX_MESSAGE_BODY_LENGTH = 4000;
+const MAX_SUBJECT_LENGTH = 200;
+
+export interface AgentDirectory {
+  getAgentName(agentId: string): string;
+}
+
+/**
+ * Strip control characters (except newlines/tabs) and collapse excessive
+ * whitespace runs. Mirrors `message-normalize.ts#cleanText` to keep the
+ * transcript resilient against garbage from upstream senders.
+ */
+function cleanText(text: string): string {
+  // eslint-disable-next-line no-control-regex
+  return text.replace(/[\x00-\x08\x0B\x0C\x0E-\x1F\x7F]/g, "")
+    .replace(/[ \t]{4,}/g, "  ")
+    .trim();
+}
+
+function truncate(text: string, maxLen: number): string {
+  if (text.length <= maxLen) return text;
+  return text.slice(0, maxLen - 1) + "…";
+}
+
+function formatChannelLabel(record: AgentMessageRecord): string | null {
+  if (record.channel === "direct") return null;
+  if (record.channelTargetId) return `${record.channel}:${record.channelTargetId}`;
+  return record.channel;
+}
+
+function resolveSenderLabel(
+  record: AgentMessageRecord,
+  directory: AgentDirectory,
+): string {
+  switch (record.senderKind) {
+    case "human":
+      // The board-reader virtual account is the only non-agent human sender
+      // surfaced in practice; render it as "Board" so agents know where
+      // strategic directives originate.
+      if (record.fromAgentId === "__board__") return "Board";
+      return "User";
+    case "system":
+      return "System";
+    case "agent":
+    default:
+      return directory.getAgentName(record.fromAgentId);
+  }
+}
+
+/**
+ * Projects a stored `AgentMessageRecord` into a pi `AgentMessage`
+ * (role: "user") suitable for appending to the recipient's session
+ * transcript. `timestamp` carries the message's `createdAt` in
+ * ms-since-epoch so transcript ordering aligns with wall-clock time.
+ */
+export function projectInboxMessageToPi(
+  record: AgentMessageRecord,
+  directory: AgentDirectory,
+): AgentMessage {
+  const senderLabel = resolveSenderLabel(record, directory);
+  const channelLabel = formatChannelLabel(record);
+  const header = channelLabel
+    ? `[Message from ${senderLabel} · ${channelLabel}]`
+    : `[Message from ${senderLabel}]`;
+
+  const subject = record.subject ? truncate(cleanText(record.subject), MAX_SUBJECT_LENGTH) : "";
+  const body = truncate(cleanText(record.body), MAX_MESSAGE_BODY_LENGTH);
+
+  const parts: string[] = [header];
+  if (subject.length > 0) parts.push(subject);
+  if (body.length > 0) parts.push(body);
+  const text = parts.join("\n\n");
+
+  const timestamp = Date.parse(record.createdAt);
+  return {
+    role: "user",
+    content: [{ type: "text", text }],
+    timestamp: Number.isFinite(timestamp) ? timestamp : Date.now(),
+  } as AgentMessage;
+}