fix(mcp): pre-compile env-var regex and unify interpolation

Remove redundant inner `import re` and regex recompilation on every call in
_interpolate_env_vars. Add module-level _ENV_VAR_PATTERN compiled once.

Replace the separate _interpolate_value() in mcp_config.py (which used \w+
and would silently fail on env vars containing hyphens or dots) with the
shared _ENV_VAR_PATTERN from mcp_tool.py. Remove now-unused import re.

hermes_cli/mcp_config.py

@@ -25,6 +25,7 @@ from hermes_cli.config import (
 )
 from hermes_cli.colors import Colors, color
 from hermes_constants import display_hermes_home
+from tools.mcp_tool import _ENV_VAR_PATTERN
 
 logger = logging.getLogger(__name__)
 
@@ -551,7 +552,7 @@ def cmd_mcp_test(args):
         for k, v in headers.items():
             if isinstance(v, str) and ("key" in k.lower() or "auth" in k.lower()):
                 # Mask the value
-                resolved = _interpolate_value(v)
+                resolved = _ENV_VAR_PATTERN.sub(lambda m: os.getenv(m.group(1), ""), v)
                 if len(resolved) > 8:
                     masked = resolved[:4] + "***" + resolved[-4:]
                 else:
@@ -581,13 +582,6 @@ def cmd_mcp_test(args):
     print()
 
 
-def _interpolate_value(value: str) -> str:
-    """Resolve ``${ENV_VAR}`` references in a string."""
-    def _replace(m):
-        return os.getenv(m.group(1), "")
-    return re.sub(r"\$\{(\w+)\}", _replace, value)
-
-
 # ─── hermes mcp login ────────────────────────────────────────────────────────
 
 def cmd_mcp_login(args):