### Summary
The generic webhook channel trusts caller-supplied identity fields (`sender`, `chat_id`) from the request body and applies authorization checks to those untrusted values. Because authentication is optional and defaults to disabled (`auth_token: None`), an attacker who can reach `POST /webhook` can spoof an allowlisted sender and choose arbitrary `chat_id` values, enabling high-risk message spoofing and potential IDOR-style session/chat routing abuse.

### Details
Relevant code paths:

- `src/channels/webhook.rs:121` sets runtime default `auth_token: None`.
- `src/config/types.rs:910` also defaults webhook config `auth_token` to `None`.
- `src/channels/webhook.rs:224` (`validate_auth`) explicitly allows requests when no token is configured.
- `src/channels/webhook.rs:128` defines `WebhookPayload` with identity fields fully controlled by caller input:
- `sender: String`
- `chat_id: String`
- `src/channels/webhook.rs:421` performs allowlist authorization using `payload.sender`.
- `src/channels/webhook.rs:433` and `src/channels/webhook.rs:434` create `InboundMessage` using untrusted `payload.sender` and `payload.chat_id`.

Why this is vulnerable:

- The system treats user-provided JSON identity as authoritative identity.
- Allowlist enforcement does not verify sender authenticity beyond that payload value.
- `chat_id` is also attacker-controlled, so routing/session association can be steered to arbitrary chats/conversations.
- If the webhook is exposed without strong upstream authn/authz controls, spoofing is straightforward.

### PoC
1. Configure the webhook channel in a vulnerable posture (common default behavior):
- `enabled = true`
- `bind_address = "0.0.0.0"` (or any reachable interface)
- `port = 9876`
- `path = "/webhook"`
- `auth_token = null` (or omitted)
- `allow_from = ["trusted-user-1"]`
- `deny_by_default = true`
2. Start ZeptoClaw.
3. Send a forged request with attacker-chosen `sender` and `chat_id`, without any `Authorization` header:

```bash
curl -i -X POST "http://127.0.0.1:9876/webhook" \
-H "Content-Type: application/json" \
--data '{
"message":"FORGED: run privileged workflow",
"sender":"trusted-user-1",
"chat_id":"victim-chat-42"
}'
```

4. Observe:
- Response is `HTTP/1.1 200 OK`.
- Message is accepted as if it originated from `trusted-user-1`.
- Message is routed under attacker-chosen `chat_id` (`victim-chat-42`).

### Impact
- Vulnerability type:
- Authentication/authorization bypass (identity spoofing)
- IDOR-style routing/control issue via attacker-chosen `chat_id`
- Affected deployments:
- Any deployment exposing the generic webhook endpoint without strict upstream authentication and identity binding.
- Security consequences:
- Forged inbound messages from spoofed trusted users.
- Bypass of allowlist intent by injecting allowlisted sender IDs in payload.
- Cross-chat/session contamination or hijacking by choosing arbitrary `chat_id`.
- Potential unauthorized downstream agent/tool actions triggered by malicious input.

CWE-306 Missing Authentication for Critical Function

CWE-345