Make Slack E2E assertions follow the intended behavior

[ReganBell]

The live Slack catalog should fail when the requested behavior fails, rather than because an otherwise valid response omits a synthetic marker, uses the channel top level, or follows an interim progress message. The file fixture uploader also wrapped a 1,336-byte PNG in a 1,556-byte multipart payload that a raw upload endpoint could store as image data.

• Judge the requested channel answers without incidental marker echoes; retain strict checks when threading is explicitly requested. Exclude stale, human, status, and unrelated-thread messages while accepting top-level thread parents.
• Make the three-person prompt explicit about its channel destination and use a stable factual audience question. Accept a real acceptance draft even when commentary acknowledges the earlier conflicting request; declines, missing drafts, and claimed sends still fail.
• Seed the history scenario with ordinary conversation and an unpredictable launch name. Wait through progress messages for that fact; approval refusals and missing facts do not pass.
• Upload exact file bytes through the supported external-upload protocol, preserving share metadata.
• Retry transient Arga status GET 5xx responses within one timeout budget. Never replay provisioning POSTs, and still require confirmed cleanup.

Validation: 14 affected tests pass; typecheck, ESLint, and upstream CI pass. Independent review and its follow-up are complete. Regression coverage includes exact PNG bytes, thread metadata, channel/thread selection, delayed history answers, transient and persistent cleanup failures, and no POST replay. Live Arga replays have passed all four originally affected cases; the final history replay passed in 61 seconds. The full final catalog is still queued/running. Alerts and release gates remain enabled.