feat(sink): per-bucket fault injection — status/latency/drop (S3)

[williamdewitt]

Part of #49 (WebhookSink epic). Slice S3. One agent run, samples/** only.

Blocked by #65.

Acceptance criteria

• Per-bucket response policy configurable at runtime: force HTTP status (e.g. 500/429), inject latency, or drop (no response) — to exercise a sender's retry/backoff/dead-lettering.
• Policy set via an API endpoint (and/or a dashboard control); default = 200 OK.
• Capture still records the hook even when a fault is injected.
• Tests prove each fault mode (status, latency, drop).

Definition of done

• Builds net10.0, 0 warnings; tests green; changes confined to samples/**.

Design: docs/design/webhook-sink-design.md (Architecture → Fault injection, S3).