swap to dedicated @coe-mirror-bot for Matrix sends

Replaces shared @intel-bot credentials with a Matrix user created
specifically for this tool. Reads Infisical keys via constants
COE_MIRROR_BOT_USER_KEY and COE_MIRROR_BOT_PASSWORD_KEY so test
assertions track the code rather than hardcoded strings.

coe_mirror.py

@@ -43,6 +43,8 @@ MATRIX_ROOM = "!MBpPtYlQYMcngtpvJt:m.mealeyfamily.com"  # General
 MATRIX_TOKEN_CACHE = LOG_DIR / "matrix-token.json"
 MATRIX_TIMEOUT = 8
 MATRIX_REFRESH_THRESHOLD_SECONDS = 1800  # refresh if <30 min left
+COE_MIRROR_BOT_USER_KEY = "COE_MIRROR_BOT_USER"
+COE_MIRROR_BOT_PASSWORD_KEY = "COE_MIRROR_BOT_PASSWORD"
 
 # Allow env var overrides for testing and deployment flexibility
 if env_pcloud := os.getenv("COE_MIRROR_PCLOUD_TARGET"):
@@ -344,18 +346,19 @@ def _fetch_creds() -> Optional[Tuple[str, str]]:
     """Fetch (user, password) from Infisical via creds. Return None on failure."""
     try:
         user = subprocess.run(
-            ["creds", "get", "MATRIX_USER", "matrix"],
+            ["creds", "get", COE_MIRROR_BOT_USER_KEY, "matrix"],
             capture_output=True, text=True, timeout=5
         )
         pw = subprocess.run(
-            ["creds", "get", "MATRIX_PASSWORD", "matrix"],
+            ["creds", "get", COE_MIRROR_BOT_PASSWORD_KEY, "matrix"],
             capture_output=True, text=True, timeout=5
         )
         if user.returncode == 0 and pw.returncode == 0:
             u, p = user.stdout.strip(), pw.stdout.strip()
             if u and p:
                 return u, p
-        logger.warning("creds returned non-zero for MATRIX_USER/MATRIX_PASSWORD")
+        logger.warning("creds returned non-zero for %s/%s",
+                       COE_MIRROR_BOT_USER_KEY, COE_MIRROR_BOT_PASSWORD_KEY)
     except FileNotFoundError:
         logger.warning("creds binary not found on PATH")
     except subprocess.TimeoutExpired: