fix(buzz-acp): accept siblings under allowlist author gate

[wpfleger96]

Problem

The inbound author gate in crates/buzz-acp/src/lib.rs decides which authors' events fire an agent turn. It had an asymmetry between its two owner-dependent modes:

• OwnerOnly accepted the owner and same-owner siblings (other bots launched by the same human, verified via their kind:0 NIP-OA auth tag through is_owner_or_sibling).
• Allowlist accepted only the owner plus the explicit pubkey list — no sibling check.

So switching an agent from owner-only to allowlist — e.g. to let an external collaborator @mention it — silently revoked recognition of same-owner teammate bots. Their @mentions hit the gate's continue and never fired a turn, producing multi-hour agent blackouts where teammate handoffs sat undelivered until a human re-poked the agent.

Fix

The Allowlist arm now accepts owner + siblings + explicit external pubkeys, using the same is_owner_or_sibling check that OwnerOnly already uses. Allowlist was meant to add external people, never to revoke same-owner team bots — this restores that intent.

The gate decision is extracted into author_allowed, which makes the policy logic unit-testable (it previously lived inline inside the long run loop). The gate call site collapses to a single await.

Tests

author_gate_tests covers the Allowlist policy decision: a same-owner sibling absent from the explicit list is accepted, an explicitly listed external pubkey is accepted, a non-sibling absent from the list is rejected, and the owner is always accepted.