Stop welding "delivered" onto "read"

The inbox projection used to call `markAgentInboxDelivered` after
appending each message to the agent's session shadow. That helper
sets `agent_messages.read_at = now()` for direct messages and
inserts into `agent_message_reads` for channel messages — the SAME
flags `listAgentInboxUnread` and the public `?unreadOnly=true` API
use to define "unread". So the moment we put a message into the
recipient's transcript, the recipient's own API call returned
nothing pending. The IC prompt told the agent to "respond to ALL
unread messages"; the API said zero unread; the agent posted a
status comment instead of replying. Every silent-employee report
traces back here.

Replay safety lives entirely on `agent_message_projections`
(installed in v46) — that ledger is keyed by `(message_id,
session_id)` and `claimProjection` already short-circuits a re-run
without touching `read_at` at all. The two concepts are now
properly separated:

  - "delivered" (we put it in front of the agent) — tracked by
    the projection ledger, owned by the runtime bridge;
  - "read" (the agent acknowledged it) — tracked by `read_at` and
    `agent_message_reads`, only flipped by the explicit POST
    /messages/{id}/read endpoint the agent calls itself.

`markAgentInboxDelivered` is removed because there is no remaining
caller and its semantics were the bug — keeping it around would
just invite the same mistake again.

src/main/database.ts

@@ -20,7 +20,6 @@ export class AppDatabase extends AppDatabaseBase {
   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 ──────────────────────────────────────────────