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/discord.rs

@@ -308,4 +308,51 @@ mod tests {
         assert!(!ch.is_user_allowed("333"));
         assert!(!ch.is_user_allowed("unknown"));
     }
+
+    #[test]
+    fn allowlist_is_exact_match_not_substring() {
+        let ch = DiscordChannel::new("fake".into(), None, vec!["111".into()]);
+        assert!(!ch.is_user_allowed("1111"));
+        assert!(!ch.is_user_allowed("11"));
+        assert!(!ch.is_user_allowed("0111"));
+    }
+
+    #[test]
+    fn allowlist_empty_string_user_id() {
+        let ch = DiscordChannel::new("fake".into(), None, vec!["111".into()]);
+        assert!(!ch.is_user_allowed(""));
+    }
+
+    #[test]
+    fn allowlist_with_wildcard_and_specific() {
+        let ch = DiscordChannel::new("fake".into(), None, vec!["111".into(), "*".into()]);
+        assert!(ch.is_user_allowed("111"));
+        assert!(ch.is_user_allowed("anyone_else"));
+    }
+
+    #[test]
+    fn allowlist_case_sensitive() {
+        let ch = DiscordChannel::new("fake".into(), None, vec!["ABC".into()]);
+        assert!(ch.is_user_allowed("ABC"));
+        assert!(!ch.is_user_allowed("abc"));
+        assert!(!ch.is_user_allowed("Abc"));
+    }
+
+    #[test]
+    fn base64_decode_empty_string() {
+        let decoded = base64_decode("");
+        assert_eq!(decoded, Some(String::new()));
+    }
+
+    #[test]
+    fn base64_decode_invalid_chars() {
+        let decoded = base64_decode("!!!!");
+        assert!(decoded.is_none());
+    }
+
+    #[test]
+    fn bot_user_id_from_empty_token() {
+        let id = DiscordChannel::bot_user_id_from_token("");
+        assert_eq!(id, Some(String::new()));
+    }
 }