security(agent): redact Slack App-Level (xapp-) tokens

The xapp-<num>-<hash> format used by Slack App-Level / Socket Mode
tokens was missing from both agent/redact.py prefix patterns and
gateway/run.py gateway secret patterns, so SLACK_APP_TOKEN values could
leak through to chat users even with security.redact_secrets enabled.

Adds an anchored xapp-\d+- pattern to both redaction paths.

agent/redact.py

@@ -76,7 +76,8 @@ _PREFIX_PATTERNS = [
     r"ghu_[A-Za-z0-9]{10,}",            # GitHub user-to-server token
     r"ghs_[A-Za-z0-9]{10,}",            # GitHub server-to-server token
     r"ghr_[A-Za-z0-9]{10,}",            # GitHub refresh token
-    r"xox[baprs]-[A-Za-z0-9-]{10,}",    # Slack tokens
+    r"xapp-\d+-[A-Za-z0-9-]{10,}",      # Slack app-Level token
+    r"xox[baprs]-[A-Za-z0-9-]{10,}",    # Slack bot/app/user tokens
     r"AIza[A-Za-z0-9_-]{30,}",          # Google API keys
     r"pplx-[A-Za-z0-9]{10,}",           # Perplexity
     r"fal_[A-Za-z0-9_-]{10,}",          # Fal.ai

gateway/run.py

@@ -147,6 +147,7 @@ _GATEWAY_RATE_LIMIT_RE = re.compile(
 _GATEWAY_SECRET_PATTERNS = (
     re.compile(r"\bsk-[A-Za-z0-9][A-Za-z0-9_\-]{12,}\b"),
     re.compile(r"\bgh[pousr]_[A-Za-z0-9_]{20,}\b"),
+    re.compile(r"\bxapp-\d+-[A-Za-z0-9\-]{20,}\b"),
     re.compile(r"\bxox[baprs]-[A-Za-z0-9\-]{20,}\b"),
     re.compile(r"\bhf_[A-Za-z0-9]{20,}\b"),
     re.compile(r"\bglpat-[A-Za-z0-9_\-]{20,}\b"),