Expose discovered MCP tools through the act allowlist (#251)

Second half of the MCP client: the tools the manager discovers become rows in
the existing act allowlist instead of a parallel capability system.

An MCP tool is encoded in the columns that already exist — cmd
["mcp",<server>,<tool>], scope mcp:<server> — so no migration, and
ProposeTool/EnableTool/DisableTool, tool.Matcher and the confirm turn need no
changes. One branch in Executor.Exec routes such a row to the manager instead
of exec, and "mcp" is never run as a binary.

Discovery only ever PROPOSES. destructive comes from the inverse of the MCP
readOnlyHint, so a tool that does not promise to be read-only inherits the
confirm turn, and enabling stays on /tools behind step-up.

Voice args are positional and MCP args are named, so CallPositional binds only
what it can defend: no required properties runs bare, and a read-only tool with
exactly one required string or number gets the tail. Everything else refuses
with ErrNeedsArgs rather than guessing. The read-only condition was learned
against the live Vikunja server: update_task requires only task_id and takes
the rest as optional, so one guessed argument blanked the fields it did not
mention. A partially-filled write destroys what it omits, so a mutating tool
never receives a guessed argument.

Also: a read-only mcp_servers IPC method and an "MCP servers" card on /tools
showing transport, target and state, with the trust level of a local target
spelled out. There is deliberately no call-a-tool IPC method and no run button,
so mutation keeps exactly one path.

Vikunja #251

deploy/mavend.json

@@ -31,6 +31,20 @@
     "cooldown": "24h"
   },
 
+  "mcp": {
+    "timeout": "15s",
+    "servers": [
+      {
+        "name": "vikunja",
+        "url": "http://192.168.1.104:9100/mcp",
+        "allow_private": true,
+        "allow_tools": ["list_projects", "list_tasks", "get_task_details", "create_task"],
+        "max_tools": 6,
+        "enabled": false
+      }
+    ]
+  },
+
   "nexus":  { "url": "http://nexus:9740" },
   "praxis": { "url": "http://praxis:8989" },
   "hexis":  { "url": "http://hexis:9741" },