[connectors] Model unattended credential survivability — warn at wiring time, not via dead letters

[serge-ivo]

Design: docs/connector-auth.md. Companion to #180.

The problem

Authorization-code OAuth encodes "the user is present and consents right now." An agent chain firing at 3am from a cron trigger has no user present.

FreeWebStore issues a 24h access token with no refresh grant (grant_types_supported: ["authorization_code"]). So even a perfect DCR+PKCE flow (#180) yields a chain that dies daily and needs a human at a browser to restart it.

Nothing in the platform models this. A subscriber can wire an interactive-only connector into a cron trigger or an agent-to-agent connection, and the failure surfaces ~24h later as deliveries piling into the dead-letter queue. The delivery outbox (migration 0058) makes that visible and replayable rather than silent — a real improvement — but replay cannot fix an expired credential, so every retry is doomed and the human still finds out late.

Whether a credential can survive unattended is a property of the connector, and the platform must know it before wiring, not after.

Scope

• Connectors declare unattended: "yes" | "refresh" | "interactive-only", derived from discovered metadata where possible (refresh_token in grant_types_supported -> refresh).
• Wiring an interactive-only connector into a cron trigger or an agent-to-agent connection warns at create time — same principle as validating connection filters at create time (a chain that looks healthy but cannot run is worse than one that fails loudly).
• Credential expiry becomes a distinct delivery outcome from a transient failure: a dead letter reading "reconnect FreeWebStore" is actionable; five backoff attempts against a 401 are noise. Do not burn the retry budget on auth failures.
• Notify before a grant lapses, not after.

Acceptance

• A connector exposes its unattended class; MCP connectors derive it from discovered metadata.
• Creating a cron trigger or connection against an interactive-only connector surfaces a warning.
• A 401/403 from an expired grant dead-letters immediately with a reconnect message rather than retrying.
• Expiring grants produce a notification ahead of expiry.
• Tests cover derivation, the wiring-time warning, and expiry-vs-transient classification.

[serge-ivo]

Shipped in e41e16e — lib/connectors/unattended.ts.

• classifyDeliveryFailure + planNextAttempt(kind): auth failures now dead-letter on the first attempt with a reconnect message. Previously a 401 burned 5 attempts across ~4.25h before dead-lettering, retrying a credential that could not recover on its own.
• unattended class derived from each connector's auth in the registry.
• Wiring-time warnings returned from createConnection.

Honest caveat, asserted in a test rather than left implicit: no connector shipped today is interactive-only, so the wiring warning currently fires for nothing. It landed first deliberately — #180's DCR flow is what would introduce the first one, and this is the guardrail that says why not to. The immediately-real half is the failure classification, which fixes live behaviour now.