fix: support internal AgentBus attachments

.project-docs/20-architecture/data-flow.md

@@ -10,6 +10,9 @@
 | ERP execution | Confirmed task | Chrome extension and logged-in ERP page | Requires unique object, page identity, ownership, and write preflight |
 | Completion evidence | ERP response/requery | Control-plane receipt and business reply | Evidence is action-specific; uncertain writes fail closed |
 | Passenger workbook | Single `.xls/.xlsx` attachment | Deterministic encrypted canonical TSV | First row ignored, second row fixed header, exact leader-contact rules |
+| WeChat roster attachment | Strict transport envelope plus one structured `payload.attachments[]` entry | Existing `awaiting_attachment` task | Explicit conversation ID wins; otherwise strict `Conversation:` supplies the fallback. Placeholder text alone never creates a task. |
+| Internal attachment download | Credential-free HTTPS URL | Bounded in-memory workbook bytes | Internal/private DNS answers are allowed; the selected address is pinned, every redirect is revalidated, and URL/host/IP/bytes are omitted from logs. |
+| Operational diagnostics | Service, request, task, parser, AgentBus, attachment, database, and cleanup stages | Structured stdout/stderr and bounded Docker logs | Correlation identifiers, codes, outcomes, and durations only; no secrets or business payloads. |
 | Confirmation export | ERP source file | Archived source plus mobile delivery artifact | Visitor XLS becomes real XLSX; other types prefer PDF |
 | Release | Editable source | `dist/release-manifest.json` and versioned artifacts | Manifest owns current hashes and filenames |
 
@@ -29,4 +32,4 @@
 
 ## Last Updated
 
-2026-08-28
+2026-08-31