docs+tests: architecture diagram, security docs, 75 new edge-case tests

README:
- Add ASCII architecture flow diagram showing all layers
- Add Security Architecture section (Layer 1: Channel Auth,
  Layer 2: Rate Limiting, Layer 3: Tool Sandbox)
- Update test count to 629

New edge-case tests (75 new):
- SecurityPolicy: command injection (semicolon, backtick, dollar-paren,
  env prefix, newline), path traversal (encoded dots, double-dot in
  filename, null byte, symlink, tilde-ssh, /var/run), rate limiter
  boundaries (exactly-at, zero, high), autonomy+command combos,
  from_config fresh tracker
- Discord: exact match not substring, empty user ID, wildcard+specific,
  case sensitivity, base64 edge cases
- Slack: exact match, empty user ID, case sensitivity, wildcard combo
- Telegram: exact match, empty string, case sensitivity, wildcard combo
- Gateway: first-match-wins, empty value, colon in value, different
  headers, empty request, newline-only request
- Config schema: backward compat (Discord/Slack without allowed_users),
  TOML roundtrip, webhook secret presence/absence

629 tests passing, 0 clippy warnings

src/gateway/mod.rs

@@ -227,6 +227,42 @@ mod tests {
         let req = "POST /webhook HTTP/1.1\r\nX-Webhook-Secret:   spaced   \r\n\r\n{}";
         assert_eq!(extract_header(req, "X-Webhook-Secret"), Some("spaced"));
     }
+
+    #[test]
+    fn extract_header_first_match_wins() {
+        let req = "POST /webhook HTTP/1.1\r\nX-Webhook-Secret: first\r\nX-Webhook-Secret: second\r\n\r\n{}";
+        assert_eq!(extract_header(req, "X-Webhook-Secret"), Some("first"));
+    }
+
+    #[test]
+    fn extract_header_empty_value() {
+        let req = "POST /webhook HTTP/1.1\r\nX-Webhook-Secret:\r\n\r\n{}";
+        assert_eq!(extract_header(req, "X-Webhook-Secret"), Some(""));
+    }
+
+    #[test]
+    fn extract_header_colon_in_value() {
+        let req = "POST /webhook HTTP/1.1\r\nAuthorization: Bearer sk-abc:123\r\n\r\n{}";
+        // split_once on ':' means only the first colon splits key/value
+        assert_eq!(extract_header(req, "Authorization"), Some("Bearer sk-abc:123"));
+    }
+
+    #[test]
+    fn extract_header_different_header() {
+        let req = "POST /webhook HTTP/1.1\r\nContent-Type: application/json\r\nX-Webhook-Secret: mysecret\r\n\r\n{}";
+        assert_eq!(extract_header(req, "Content-Type"), Some("application/json"));
+        assert_eq!(extract_header(req, "X-Webhook-Secret"), Some("mysecret"));
+    }
+
+    #[test]
+    fn extract_header_from_empty_request() {
+        assert_eq!(extract_header("", "X-Webhook-Secret"), None);
+    }
+
+    #[test]
+    fn extract_header_newline_only_request() {
+        assert_eq!(extract_header("\r\n\r\n", "X-Webhook-Secret"), None);
+    }
 }
 
 async fn send_json(