fix(handlers): timestamp_outside_window agent_action contract — add URL

TestAgentActionContract/codeToAgentAction[timestamp_outside_window]
failed on master CI because the agent_action string registered in
1ab09a2 lacked the required https://instanode.dev/ URL. Rule 4 of
the contract test (every agent_action MUST contain a full HTTPS URL)
was missed when the registry entry was added.

Adds `— see https://instanode.dev/docs/webhooks.` to the agent_action
string. Length 220 → 252 chars (still well below the 280 ceiling).

Verified locally: TestAgentActionContract/codeToAgentAction[timestamp_outside_window]
now passes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: mastermanas805

internal/handlers/helpers.go

@@ -409,7 +409,7 @@ var codeToAgentAction = map[string]errorCodeMeta{
 		AgentAction: "Tell the user the webhook signature did not verify. Confirm the webhook secret in your dashboard and retry — see https://instanode.dev/docs/webhooks.",
 	},
 	"timestamp_outside_window": {
-		AgentAction: "Tell the user the webhook timestamp is outside the accepted ±5-minute window. Stop replaying captured webhook payloads — retry with a fresh delivery. If clock skew is suspected, sync the sender's clock via NTP.",
+		AgentAction: "Tell the user the webhook timestamp is outside the accepted ±5-minute window. Stop replaying captured webhook payloads — retry with a fresh delivery. If clock skew is suspected, sync the sender's clock via NTP — see https://instanode.dev/docs/webhooks.",
 	},
 	"signature_invalid": {
 		AgentAction: "Tell the user the request signature failed verification. Confirm the signing key and the canonical request body and retry — see https://instanode.dev/docs.",