[Slack] Fail closed unless a writeback receipt contains a real provider timestamp

[khaliqgant]

Problem

The shared Slack receipt helper currently returns externalId ?? ts ?? created ?? id ?? "", and adapter-core only checks whether externalId or ts is non-empty. This creates two fleet-wide false-success paths:

• receipt.created or receipt.id can be mistaken for the delivered Slack message timestamp.
• A blank or whitespace externalId can hide a valid ts, while any non-empty non-Slack-shaped value is accepted.

This is the same failure class independently fixed at persona boundaries in relay#1332 and internal-agents#25: a run can report Slack success without a real provider message timestamp.

Current shared owners:

• packages/relay-helpers/src/slack.ts (slackReceiptTs)
• packages/core/src/vfs-client/index.ts (hasSlackReceiptTs / terminal receipt validation)
• any mounted-mode status/parser path that promotes receipt metadata to delivery success

Required contract

A Slack delivery is confirmed only when the receipt contains a trimmed Slack-ts-shaped value such as 1733512345.001900.

1. Independently trim and validate externalId, then ts.
2. Accept the first value matching the Slack timestamp contract (at minimum ^\\d{10,}\\.\\d+$).
3. Never fall back to created or id as a Slack message timestamp.
4. Make adapter-core terminal validation use the same shared predicate so mounted-mode callers cannot report success on metadata-only receipts.
5. Keep the contract in one exported helper/predicate rather than duplicating it per persona.

Acceptance tests

• valid externalId succeeds and is returned trimmed
• blank externalId plus valid ts succeeds via ts
• created/id-only fails closed
• whitespace-only externalId/ts fails closed
• non-Slack-shaped externalId/ts fails closed
• mounted adapter-core terminal receipt validation rejects the same invalid cases
• existing valid direct and idempotency-replay receipts remain green

Follow-up from the 2026-07-18 fleet recovery; this should not block the already reviewed persona rollout PRs.

[khaliqgant]

Cross-repository follow-through is tracked in cloud#2742. This issue is the canonical implementation owner for the shared predicate and adapter-core validation; the Cloud issue should consume the published adapter version and verify the production mounted-mode behavior, not reimplement the grammar.