feat: add skill=unknown to allowed skills for MCP proxy mode

MCP proxy is skill-agnostic per call — the CLI subprocess doesn't know
which skill triggered the request. Adding "unknown" to the skill
allowlist enables consistent 3-key context (app/skill/agent) across all
channels instead of omitting skill for MCP.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: arorashivam96

src/PowerPlatform/Dataverse/core/config.py

@@ -35,6 +35,7 @@
         "dv-solution",
         "dv-admin",
         "dv-security",
+        "unknown",
     }
 )
 _ALLOWED_AGENTS = frozenset(