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

cmd/mavend/main.go

@@ -280,6 +280,9 @@ func run(args []string) error {
 			api := coreAPI.(*daemonAPI)
 			api.chatFn = voiceW.handler.handleText
 		}
+		if voiceW != nil && voiceW.mcp != nil {
+			coreAPI.(*daemonAPI).getMCPServers = voiceW.mcp.status
+		}
 	} else {
 		// locked mode: no real store yet, so there's no meaningful CoreAPI to
 		// serve. srv.Check below is the actual guard — every CoreAPI call is
@@ -518,6 +521,11 @@ func run(args []string) error {
 				}()
 			}
 
+			// Keep MCP connections alive (nil unless configured).
+			if voiceW != nil && voiceW.mcp != nil {
+				go voiceW.mcp.run(ctx)
+			}
+
 			dl.unlock()
 			log.Printf("mavend: unlocked via passkey assertion")
 			return nil
@@ -577,6 +585,13 @@ func run(args []string) error {
 				crawlWkr.run(ctx)
 			}()
 		}
+		if voiceW != nil && voiceW.mcp != nil {
+			wg.Add(1)
+			go func() {
+				defer wg.Done()
+				voiceW.mcp.run(ctx)
+			}()
+		}
 	}
 
 	<-ctx.Done()