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/channels/slack.rs

@@ -209,4 +209,31 @@ mod tests {
         assert!(ch.is_user_allowed("U222"));
         assert!(!ch.is_user_allowed("U333"));
     }
+
+    #[test]
+    fn allowlist_exact_match_not_substring() {
+        let ch = SlackChannel::new("xoxb-fake".into(), None, vec!["U111".into()]);
+        assert!(!ch.is_user_allowed("U1111"));
+        assert!(!ch.is_user_allowed("U11"));
+    }
+
+    #[test]
+    fn allowlist_empty_user_id() {
+        let ch = SlackChannel::new("xoxb-fake".into(), None, vec!["U111".into()]);
+        assert!(!ch.is_user_allowed(""));
+    }
+
+    #[test]
+    fn allowlist_case_sensitive() {
+        let ch = SlackChannel::new("xoxb-fake".into(), None, vec!["U111".into()]);
+        assert!(ch.is_user_allowed("U111"));
+        assert!(!ch.is_user_allowed("u111"));
+    }
+
+    #[test]
+    fn allowlist_wildcard_and_specific() {
+        let ch = SlackChannel::new("xoxb-fake".into(), None, vec!["U111".into(), "*".into()]);
+        assert!(ch.is_user_allowed("U111"));
+        assert!(ch.is_user_allowed("anyone"));
+    }
 }